Synthetic Proppants And Monodispersed Proppants And Methods Of Making The Same

ABSTRACT

Synthetic ceramic proppants are described. Proppants having a monodispersity of 3-sigma distribution or lower are also described, including methods to make these proppants and methods of using these proppants.

BACKGROUND OF THE INVENTION

This application claims the benefit under 35 U.S.C. §119(e) of priorU.S. Provisional Patent Application No. 61/678,318, filed Aug. 1, 2012,which is incorporated in its entirety by reference herein.

The present invention relates to proppants and methods of makingproppants. The present invention further relates to the use of proppantsfor hydrocarbon recovery. The present invention further relates to themethod of fracking a well using the proppants of the present invention.

Proppants are materials pumped into oil or gas wells at extreme pressurein a carrier solution (typically brine) during the hydrofracturingprocess. Once the pumping-induced pressure is removed, proppants “prop”open fractures in the rock formation and thus preclude the fracture fromclosing. As a result, the amount of formation surface area exposed tothe well bore is increased, enhancing recovery rates.

Ceramic proppants are widely used as propping agents to maintainpermeability in oil and gas formations. High strength ceramic proppantshave been used in the hydrofracture of subterranean earth in order toimprove production of natural gas and/or oil. For wells that are drilled10,000 feet or deeper into the earth, the proppant beads need towithstand 10 kpsi or higher pressure to be effective to prop thefracture generated by the hydrofracture process. Currently onlyproppants formed from high strength materials, such as sintered bauxiteand alumina have sufficient compressive and flexural strength for use indeep wells. These conventional high strength materials are expensive,however, because of a limited supply of raw materials, a highrequirement for purity, and the complex nature of the manufacturingprocess. In addition, such high strength materials have high specificgravity, in excess of 3.0, which is highly undesirable for proppantapplications. Producing high strength proppants with low specificgravity is also a challenge. In field applications, the transportabilityof proppants in wells is hindered by the difference of specificgravities of proppant and carrying fluid. While light weight oxidematerials, such as cordierite, have low specific gravity, they have arelatively weak flexural strength and stiffness.

While ceramic proppants have been known, the previous ceramic proppantsthat are considered conventional had numerous defects andinconsistencies. For instance, as can be seen in FIGS. 21 and 22,conventional proppants were not uniform in shape or in surfacecharacteristics. This is further confirmed by various ceramic proppantspreviously described or commercially available. For instance, FIGS.26-31 provide images of various conventional ceramic proppants, and, ascan be seen from these images, the surface of the proppants had numerousdefects with regard to irregular and inconsistent shapes, irregular andinconsistent sizes, or surface defects. Each of these negativeattributes would lead to inconsistent proppant performance when injectedinto a well and most especially would lead to proppant failure at a lowcrush strength.

While there is literature that describes nearly-monodispersed proppantsand other references that characterize particles or proppants asmonodispersed, there is a problem with such characterizations. First, noquantified descriptions are given when the term “monodispersed” is usedto characterize particles of proppants. Thus, the monodispersity mayhave an immense distribution area involved, such that the standarddeviation is over five standard deviations. No effort has been made inmost, if not all, of this literature to quantify the monodispersity.Further, based on the methods described in these various literaturearticles, it would appear that achieving a highly-monodispersed proppantpopulation would not be possible and that the standard deviation wouldbe significant.

In addition, while various methods can be used to make proppants, andthen classification techniques can be used to achieve some standardsizing, it is important to point out the following. Standard screen orsieve classifications will have typically a deviation or error of ±100microns, for instance. The coefficient of variation for screen or sieveclassification is over 20 to 25% or higher, whereas the coefficient ofvariation for air classification methods would be a coefficient ofvariation of 10 to 15% or higher. None of these techniques would producea proppant population of monodispersity and further would not create aproppant population with a 3-sigma distribution with the width of thetotal distribution being more than 5% of the mean particle size.

Accordingly, there is a need to provide proppant populations of ceramicproppants, wherein the ceramic proppants are monodispersed with a3-sigma distribution or better. In addition, there is a need to providemethods which can achieve this monodispersity.

SUMMARY OF THE PRESENT INVENTION

A feature of the present invention is to provide a plurality ofproppants having high monodispersity.

A further feature of the present invention is to provide a proppantpopulation that comprises, consists essentially of, or consists ofceramic proppants, wherein the proppants are monodispersed.

An additional feature of the present invention is to provide methods tomake a plurality of proppants, such as ceramic proppants, which havemonodispersity and can optionally be achieved without the need for anypost-classification processing.

A further feature of the present invention is to provide new methods tomake ceramic core/shell proppants where the core can include a hollowportion that is created during sintering of a solid green body core anda solid green body shell.

Further, a feature of the present invention is to provide proppantshaving a balance of strength properties from the shell and the core.

Additional features and advantages of the present invention will be setforth in part in the description that follows, and in part will beapparent from the description, or may be learned by practice of thepresent invention. The objectives and other advantages of the presentinvention will be realized and attained by means of the elements andcombinations particularly pointed out in the description and appendedclaims.

To achieve these and other advantages, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the present invention relates to a plurality of sintered ceramicproppants, wherein the sintered ceramic proppants are monodispersed witha 3-sigma distribution or better with the width of the totaldistribution being 5% or less of the mean particle size. The pluralityof ceramic proppants can be considered a population of proppants.

The present invention also relates to a plurality of ceramic proppantsthat can be green ceramic proppants or sintered ceramic proppants orboth, having a Krumbein sphericity of at least 0.9, such as at least0.94 or 0.95 to 0.99.

Also, the present invention relates to a proppant having a plurality ofmicron particles that are sintered together, wherein the micronparticles (prior to sintering) have a unimodal particle distribution,bi-modal particle distribution, or a multi-modal particle distribution(e.g., tri-modal particle distribution).

Furthermore, the present invention relates to a proppant having a coreand a shell, wherein the core is ceramic or an oxide thereof and theshell comprises at least one ceramic material, wherein the proppant hasa core strength to shell strength ratio of from 0.8 to 1. The proppantcan have an overall proppant strength to core strength ratio of from 2to 3.

The present invention further relates to methods of making the proppantsof the present invention. The method can include preparing a slurry ofceramic particles and/or ceramic precursor particles, and then forming aplurality of green bodies by spray-drying the slurry into sphericalgreen bodies (or other shapes) which can then optionally be coated withone or more coatings or shell-forming materials of the same or differentmaterial used to form the green body. The optional coating technique canbe achieved by fluid bed coating techniques, and then the green body issintered. The ceramic proppants (in the green state (before or after theoptional shell) and/or in the sintered state) can have a monodispersitywith a 3-sigma distribution or lower (e.g., 2-sigma or 1-sigmadistribution) with the width of the total distribution being 5% or lessof the mean particle size. Monodispersity can be achieved, for instance,using specially designed spray heads attached to the spray dryer asdescribed herein. The monodispersity can be achieved using variousspray-dryer settings and parameters as described herein.

The present invention further relates to a method to prop opensubterranean formation fractures by utilizing the proppants of thepresent invention. The proppant population of the present invention canbe combined with one or more fluids to form a suspension, which can thenbe pumped into the subterranean producing zone. Further details areprovided herein.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide a further explanation of the presentinvention, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this application, illustrate some of the features of the presentinvention and together with the description, serve to explain theprinciples of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a SEM image of microspheres from an example of the presentapplication.

FIG. 2 is a SEM image of higher magnification, compared to FIG. 1, ofmicrospheres of the present invention.

FIG. 3 is a further enlarged SEM image of a microsphere of the presentinvention.

FIG. 4 is a graph showing particle size distribution versus inlettemperature and the effects achieved by adjusting the inlet temperatureof the spray dryer.

FIG. 5 is a SEM image of a 40/50 proppant of the present invention.

FIG. 6 is a SEM image showing a 30/40 proppant of the present invention.

FIG. 7 is a drawing of an exposed side view of a spray nozzle that canbe used in the methods of the present invention.

FIG. 8 is a diagram of a proppant (enlarged) that shows the schematicsof void formation in the center of the proppant in the core region dueto the partial or complete diffusion of the core material from the greenbody and further shows the diffusion or migration of the core materialinto the shell regions. FIG. 8 shows that the diffusion of the corematerial forms a type of gradient and, therefore, a higher concentrationof core material is present closer to the core than the outer surface ofthe proppant, with migration or diffusion of the core material occurringin an outward radial direction. FIG. 8 also comprises three graphs thatshow the degree of porosity, core material concentration, and mullitewhisker formation/concentration based on location within the proppant.The three graphs are in alignment with the location shown in theproppant sphere diagram or drawing.

FIG. 9 is a SEM image showing 40/50 mesh green proppant fabricated fromsynthetic templates of the present invention.

FIG. 10 is a SEM image showing 30/40 mesh green proppant fabricated fromsynthetic templates of the present invention.

FIGS. 11 and 12 are graphs showing particle size distribution of eithera 40/50 green body proppant or a 30/40 green body proppant and the tightparticle size distributions achievable, including a small Sigma 3 value.

FIG. 13 is a SEM image showing a proppant of the present invention,wherein Region A is the interface between the core and shell, Region Bis the central location of the shell, and Region C is the outer regionof the shell near the surface of the overall proppant.

FIGS. 14-16 are magnified images of Regions A, B, and C, respectively.

FIGS. 17-20 are SEM images which show the progression of the diffusionof the core (partial or complete) into the shell regions of a proppantthrough sintering/diffusion kinetics.

FIG. 21 is a SEM image of a conventional pre-formed cenosphere that haspreviously been used as a template in proppants.

FIG. 22 is a SEM image showing a cross-section of one of the cenospheresfrom FIG. 21, and showing various structural defects and non-uniformityin shape and size.

FIG. 23 is a SEM image of synthetic templates of the present inventionwhich, in this example, were formed by spray-drying techniques andsintered at 1025° C.

FIG. 24 is a SEM image of one of the synthetic templates from FIG. 23.As can be seen, more uniformity and low defects are shown in thisfigure, especially compared to FIG. 22. This figure shows a sinteredsolid synthetic template.

FIG. 25 is a SEM image of a synthetic template of the present invention,which is hollow in the central position of the sphere.

FIG. 26 is a SEM image of a conventional ceramic proppant, particularlya James Hardie cenosphere.

FIG. 27 is an image from a Department of Energy publication by Cutler etal. showing spray-dried ceramic proppants.

FIG. 28 is a SEM image of ceramic proppants that are James Hardiecenospheres.

FIG. 29 is a SEM image of conventional ceramic proppants from Kerabims.

FIG. 30 is a SEM image of ceramic proppants that are known as Macroliteproppants.

FIG. 31 is a SEM image of conventional ceramic proppants known asPoraver proppants.

FIG. 32 is a schematic which shows one proppant design of the presentinvention.

FIG. 33 is a SEM image of the polymeric templates used for proppantpreparation.

FIG. 34 is a SEM image of the cross-section of a hollow ceramicsynthetic template made from spray coating on the polymeric core shownin FIG. 33 followed by burnout of the polymeric core and sintering ofthe shell.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to a proppant, populations of proppants,methods of making the proppants, and uses for the proppants, includingusing the proppants in hydrocarbon recovery.

The present invention makes it possible to achieve a population ofceramic proppants, more specifically, green and/or sintered ceramicproppants, wherein the green or sintered ceramic proppants can bemonodispersed with a 3-sigma distribution or lower with the width of thetotal distribution being 5% or less of the mean particle size (which canbe considered a 5% tolerance). In other words, the plurality of theproppants or the population of the proppants can be highly monodispersedand have a standard deviation of 3 or less. Standard deviation can be 3or less, 2.75 or less, 2.5 or less, 2.25 or less, 2 or less, 1.75 orless, 1.5 or less, 1.25 or less, 1 or less, 0.9 or less, 0.8 or less,0.7 or less, 0.6 or less, or 0.5 or less standard deviation. Thestandard deviation can be from 1 to 3, 0.5 to 3, 0.5 to 2.5, 0.5 to 2and the like. Put another way, the population of green or sinteredceramic proppants of the present invention can be with a 3-sigmadistribution, can be with a 2-sigma distribution, or can be with a1-sigma distribution with the width of the total distribution being 5%or less of the mean particle size (which can be considered a 5%tolerance).

The proppants of the present invention can have a coefficient ofvariation (or coefficient of variance) (CV) of 8% or less, such as fromabout 5% to about 8%. This coefficient of variation can apply to thegreen body core, the green body that comprises a core and shell, and/orthe sintered proppant resulting from these green bodies. The coefficientof variation is also known as the coefficient of variance. For purposesof the present invention, the coefficient of variance or coefficient ofvariation is calculated by:

$\frac{\left( {{standard}\mspace{14mu} {deviation}\mspace{14mu} {in}\mspace{14mu} {microns}} \right)}{\left( {{mean}\mspace{14mu} {particle}\mspace{14mu} {size}\mspace{14mu} {in}\mspace{14mu} {microns}} \right)}.$

As stated above, this highly monodispersed population of proppants ofthe present invention is not achieved through conventionalclassification techniques, such as screen or sieve classifying or airclassifying. One way to achieve such a highly monodispersed proppant isthrough manufacturing techniques which form the green and/or sinteredceramic proppant in the desired shape and in an extremely consistentmariner.

For purposes of the present invention, a ceramic proppant is a proppantthat contains at least 90% by weight ceramic materials based on theentire weight of the ceramic proppant. For example, the ceramic proppantcan contain at least 92% by weight ceramic materials, at least 95% byweight ceramic materials, at least 96% by weight ceramic materials, atleast 97% by weight ceramic materials, at least 98% by weight ceramicmaterials, at least 99% by weight ceramic materials, at least 99.5% byweight ceramic materials, at least 99.9% by weight ceramic materials, orcan be 100% by weight ceramic materials. The ceramic materials, forpurposes of the present invention, can be one or more metal oxides,and/or one or more non-oxides that are considered ceramics, such ascarbides, borides, nitrides, and/or silicides. For purposes of thepresent invention, the term “ceramic” includes glass material, ceramicmaterial, and/or glass-ceramic material and/or can comprise one or moreglass, ceramic, and/or glass-ceramic phases. The “ceramic” material canbe non-crystalline, crystalline, and/or partially crystalline.

For purposes of the present invention, the ceramic proppant can haveless than 5 wt % polymeric and/or cellulosic (e.g., plant material ortree material). More preferably, the proppants of the present inventionhave less than 1 wt %, less than 0.5 wt %, less than 0.1 wt %, or 0 wt %of polymeric material or cellulosic material or both in the sinteredproppants of the present invention.

The ceramic in the ceramic proppants of the present invention can be anoxide, such as aluminum oxides (alumina) and/or mixed metal aluminumoxides, such as metal aluminates containing calcium, yttrium, titanium,lanthanum, barium, and/or silicon in addition to aluminum. The ceramiccan be an oxide, such as aluminum oxide called alumina, or a mixed metaloxide of aluminum called an aluminate, a silicate, or analuminosilicate, such as mullite or cordierite. The aluminate or theceramic in general may contain magnesium, calcium, yttrium, titanium,lanthanum, barium, and/or silicon. The ceramic may be formed from ananoparticle precursor such as an alumoxane. Alumoxanes can bechemically functionalized aluminum oxide nanoparticles with surfacegroups including those derived from carboxylic acids such as acetate,methoxyacetate, methoxyethoxyacetate, methoxyethoxyethoxyacetate,lysine, and stearate, and the like. The ceramic can include, but is notlimited to, boehmite, alumina, spinel, alumnosilicate clays (e.g.,kaolin, montmorillonite, bentonite, and the like), calcium carbonate,calcium oxide, magnesium oxide, magnesium carbonate, cordierite, spinel,spodumene, steatite, a silicate, a substituted alumino silicate clay orany combination thereof (e.g. kyanite) and the like.

The ceramic can be or contain cordierite, mullite, bauxite, silica,spodumene, clay, silicon oxide, aluminum oxide, sodium oxide, potassiumoxide, calcium oxide, zirconium oxide, lithium oxide, iron oxide,spinel, steatite, a silicate, a substituted alumino silicate clay, aninorganic nitride, an inorganic carbide or a non-oxide ceramic or anymixtures thereof. The proppant can include or be one or more sedimentaryand/or synthetically produced materials.

Glass-ceramic, as used herein, refers to any glass-ceramic that isformed when glass or a substantially glassy material is annealed atelevated temperature to produce a substantially crystalline material,such as with limited crystallinity or controlled crystallite size. Asused herein, limited crystallinity should be understood as crystallinityof from about 5% to about 100%, by volume (e.g., 10% to 90%; 20% to 80%;30% to 70%; 40% to 60% by volume). The crystallite size can be fromabout 0.01 micrometers to 20 micrometers, such as 0.1 to 5 micrometers.Preferably the crystallite size is less than 1 micrometer. Theglass-ceramic can be composed of aluminum oxide, silicon oxide, boronoxide, potassium oxide, zirconium oxide, magnesium oxide, calcium oxide,lithium oxide, phosphorous oxide, and/or titanium oxide or anycombination thereof.

The glass-ceramic can comprise from about 35% to about 55% by weightSiO₂; from about 18% to about 28% by weight Al₂O₃; from about 1% toabout 15% by weight (e.g., 1 to 5 wt %) CaO; from about 7% to about 14%by weight MgO; from about 0.5% to about 15% by weight TiO₂ (e.g., 0.5 to5 wt %); from about 0.4% to about 3% by weight B₂O₃, and/or greater than0% by weight and up to about 1% by weight P₂O₅, all based on the totalweight of the glass-ceramic. The glass-ceramic can comprise from about3% to about 5% by weight Li₂O; from about 0% to about 15% by weightAl₂O₃; from about 10% to about 45% by weight SiO₂; from about 20% toabout 50% by weight MgO; from about 0.5% to about 5% by weight TiO₂;from about 15% to about 30% by weight B₂O₃, and/or from about 6% toabout 20% by weight ZnO, all based on the total weight of theglass-ceramic.

The proppant can comprise aluminum oxide, silicon oxide, titanium oxide,iron oxide, magnesium oxide, calcium oxide, potassium oxide and/orsodium oxide, and/or any combination thereof. The sintered proppant canbe or include at least in part cordierite, mullite, bauxite, silica,spodumene, silicon oxide, aluminum oxide, sodium oxide, potassium oxide,calcium oxide, zirconium oxide, lithium oxide, iron oxide, spinel,steatite, a silicate, a substituted alumino silicate clay, an inorganicnitride, an inorganic carbide, a non-oxide ceramic or any combinationthereof.

The glass-ceramic proppant can be fully or nearly fully crystalline orcan contain a glass component (e.g., phase(s)) and a crystallinecomponent (e.g., phase(s)) comprising crystallites. The glass-ceramiccan have a degree of crystallinity of from about 5% to about 100%, orfrom about 15% to about 80%. For example, the glass-ceramic can havefrom about 50% to 80% crystallinity, from about 60% to 78% crystallinityor from about 70% to 75% crystallinity by volume. The crystallites canhave a random and/or directed orientation. With respect to theorientation of the crystals that are present in the glass-ceramic, thecrystal orientation of the crystals in the glass-ceramic can beprimarily random or can be primarily directed in a particularorientation(s) (e.g., non-random). For instance, the crystal orientationof the glass-ceramic can be primarily random such that at least 50% orhigher of the orientations are random orientations based on the overallorientation of the crystals present. For instance, the randomorientation can be at least 60%, at least 70%, at least 80%, at least90%, such as from about 51% to 99%, from 60% to 90%, from 70% to 95% orhigher with respect to the percent of the crystals that are random basedon the crystals measured. X-ray diffraction (“XRD”) can be used todetermine the randomness of the crystallites. As the glass-ceramic canhave both crystal and glass components, the glass-ceramic can havecertain properties that are the same as glass and/or crystallineceramics. Thus, the glass-ceramic can provide an ideal gradientinterface between the template sphere and the ceramic shell, if present.The glass-ceramic can be impervious to thermal shock. Furthermore, theproportion of the glass and crystalline component of the glass-ceramiccan be adjusted to match (e.g., within 10%, within 5%, within 1%, within0.5%, within 0.1%) the coefficient of thermal expansion (CTE) of theshell (if present) or other material to which it will be bonded orattached or otherwise in contact with, in order to prevent prematurefracture(s) resulting from cyclic stresses due to temperature changes,or thermal fatigue. For example, when the glass-ceramic has from 70% to78% crystallinity, the two coefficients balance such that theglass-ceramic as a whole has a thermal expansion coefficient mismatchthat is very close to zero.

Glass (which can be considered a ceramic type of material), as usedherein, can be any inorganic, non-metallic solid non-crystallinematerial, such as prepared by the action of heat and subsequent cooling.The glass can be any conventional glass such as, for example, soda-limeglass, lead glass, or borosilicate glass. Crystalline ceramic materials,as used herein, can be any inorganic, non-metallic solid crystallinematerial prepared by the action of heat and subsequent cooling. Forexample, the crystalline ceramic materials can include, but are notlimited to, alumina, zirconia, stabilized zirconia, mullite, zirconiatoughened alumina, spinel, aluminosilicates (e.g., mullite, cordierite),perovskite, perchlorate, silicon carbide, silicon nitride, titaniumcarbide, titanium nitride, aluminum oxide, silicon oxide, zirconiumoxide, stabilized zirconium oxide, aluminum carbide, aluminum nitride,zirconium carbide, zirconium nitride, iron carbide, aluminum oxynitride,silicon aluminum oxynitride, aluminum titanate, tungsten carbide,tungsten nitride, steatite, and the like, or any combination thereof.

The proppant can have a crystalline phase and a glass (or glassy) phase,or amorphous phase. The matrix or amorphous phase can include asilicon-containing oxide (e.g., silica) and/or an aluminum-containingoxide (e.g., alumina), and optionally at least one iron oxide;optionally at least one potassium oxide; optionally at least one calciumoxide; optionally at least one sodium oxide; optionally at least onetitanium oxide; and/or optionally at least one magnesium oxide, or anycombinations thereof. The matrix or amorphous phase can contain one ormore, or all of these optional oxides in various amounts where,preferably, the silicon-containing oxide is the major component byweight in the matrix and/or the amorphous phase, such as where thesilicon-containing oxide is present in an amount of at least 50.1% byweight, at least 75% by weight, at least 85% by weight, at least 90% byweight, at least 95% by weight, at least 97% by weight, at least 98% byweight, at least 99% by weight (such as from 75% by weight to 99% byweight, from 90% by weight to 95% by weight, from 90% by weight to 97%by weight) based on the weight of the matrix or based on the weight ofthe amorphous phase alone. Exemplary oxides that can be present in theamorphous phase include, but are not limited to, SiO₂, Al₂O₃, Fe₂O₃,Fe₃O₄, K₂O, CaO, Na₂O, TiO₂, and/or MgO. It is to be understood that,for purposes of the present invention, other metals and/or metal oxidescan be present in the matrix or amorphous phase.

The amorphous phase can include or be ceramic, and for instance caninclude alumina and/or silica. The amorphous phase can further includeunreacted material (e.g., particles), such as alumina, aluminaprecursor, and/or siliceous material or any combination thereof.

The proppant can include one or more minerals and/or ores, one or moreclays, and/or one or more silicates, and/or one or more solid solutions.The minerals or ores can be aluminum-containing minerals or ores and/orsilicon-containing minerals or ores. These minerals, ores, clays,silicates, and/or solid solutions can be present as particulates. Thesecomponent(s) can be present as at least one crystalline particulatephase that can be a non-continuous phase or continuous phase in thematerial. More specific examples include, but are not limited to,alumina, aluminum hydroxide, bauxite, gibbsite, boehmite or diaspore,ground cenospheres, fly ash, unreacted silica, silicate materials,quartz, feldspar, zeolites, bauxite and/or calcined clays. Thesecomponents in a combined amount can be present in the material in anamount, for instance, of from 0.001 wt % to 85 wt % or more, such asfrom 1 wt % to 80 wt %, 5 wt % to 75 wt %, 10 wt % to 70 wt %, 15 wt %to 65 wt %, 20 wt % to 60 wt %, 30 wt % to 70 wt %, 40 wt % to 70 wt %,45 wt % to 75 wt %, 50 wt % to 70 wt %, 0.01 wt % to 10 wt %, 0.1 wt %to 8 wt %, 0.5 wt % to 5 wt %, 0.75 wt % to 5 wt %, 0.5 wt % to 3 wt %,0.5 wt % to 2 wt % based on the weight of the material. These amountsand ranges can alternatively apply to one crystalline particulate phase,such as alumina or an aluminum-containing material. These additionalcomponents can be uniformly dispersed throughout the matrix or amorphousphase (like filler is present in a matrix as discrete particulates).

The proppant can have any particle size. For instance, the proppant canhave a particle diameter size of from about 75 microns to 1 cm or adiameter in the range of from about 100 microns to about 2 mm, or adiameter of from about 100 microns to about 3,000 microns, or a diameterof from about 100 microns to about 1,000 microns. Other particle sizescan be used. Further, the particle sizes as measured by their diametercan be above the numerical ranges provided herein or below the numericalranges provided herein.

The proppant can have any median particle size, such as a medianparticle size, d_(p50), of from about 90 μm to about 2000 μm (e.g., from90 μm to 2000 μm, from 100 μm to 2000 μm, from 200 μm to 2000 μm, from300 μm to 2000 μm, from 500 μm to 2000 μm, from 750 μm to 2000 μm, from100 μm to 1000 μm, from 100 μm to 750 μm, from 100 μm to 500 μm, from100 μm to 250 μm, from 250 μm to 2000 μm, from 250 μm to 1000 μm),wherein d_(p50) is a median particle size where 50% of the particles ofthe distribution have a smaller particle size.

The proppants of the present application can, for instance, have aspecific gravity of from about 0.6 g/cc to about 4 g/cc. The specificgravity can be from about 1.0 g/cc to about 3 g/cc or can be from about0.9 g/cc to about 2.5 g/cc, or can be from 1.0 g/cc to 2.5 g/cc, or from1.0 g/cc to 2.4 g/cc, or from 1.0 g/cc to 2.3 g/cc, or from 1.0 g/cc to2.2 g/cc, or from 1.0 g/cc to 2.1 g/cc, or 1.0 g/cc to 2.0 g/cc. Otherspecific gravities above and below these ranges can be obtained. Theterm “specific gravity” as used herein is the weight in grams per cubiccentimeter (g/cc) of volume, excluding open porosity in determining thevolume. The specific gravity value can be determined by any suitablemethod known in the art, such as by liquid (e.g., water or alcohol)displacement or with a gas pycnometer.

The proppant (green body and/or sintered proppant) can be spherical andhave a Krumbein sphericity of at least about 0.5, at least 0.6 or atleast 0.7, at least 0.8, or at least 0.9, and/or a roundness of at least0.4, at least 0.5, at least 0.6, at least 0.7, or at least 0.9. The term“spherical” can refer to roundness and sphericity on the Krumbein andSloss Chart by visually grading 10 to 20 randomly selected particles. Asan option, in the present invention, the proppants of the presentinvention can have a very high degree of sphericity. In particular, theKrumbein sphericity can be at least 0.92, or at least 0.94, such as from0.92 to 0.99, or from 0.94 to 0.99, or from 0.97 to 0.99, or from 0.95to 0.99. This is especially made possible by the methods of the presentinvention, including forming synthetic templates on cores and using aspray dryer or similar device.

With regard to the proppant (either in the green body state or as asintered proppant or both), the proppant has a change in sphericity of5% or less. This change in sphericity parameter is with respect to theproppant (either in the green body state or sintered proppant state) inthe shape of a sphere and this change in sphericity parameter refers tothe uniformity of the sphere around the entire surface area of theexterior of the sphere. Put another way, the curvature that defines thesphere is very uniform around the entire sphere such that the change insphericity compared to other points of measurement on the same spheredoes not change by more than 5%. More preferably, the change insphericity is 4% or less or 3% or less, such as from about 0.5% to 5% orfrom about 1% to about 5%.

The proppants of the present invention can have a crush strength of1,000 psi to 20,000 psi or higher (e.g., from 1,500 psi to 10,000 psi,from 3,000 psi to 10,000 psi, from 5,000 psi to 10,000 psi, from 9,000psi to 12,000 psi). Other crush strengths below or above these rangesare possible. Crush strength can be measured, for example, according toAmerican Petroleum Institute Recommended Practice 60 (RP-60) oraccording to ISO 13503-2.

The proppant can have a flexural strength in a range of from about 1 MPato about 800 MPa, or more, such as 1 MPa to 700 MPa, 5 MPa to 600 MPa,10 MPa to 500 MPa, 25 MPa to 400 MPa, 50 MPa to 200 MPa, and the like.

The proppant or part thereof can have a coefficient of thermal expansion(CTE at from 25° C. to 300° C.) of from about 0.1×10⁻⁶/K to about13×10⁻⁶/K, such as from 0.1×10⁻⁶/K to 2×10⁻⁶/K or 1.2×10⁻⁶/K to1.7×10⁻⁶/K. The proppant can have a MOR of from about 1 to about 800MPa, such as 100 to 500 MPa.

The present invention further relates to a proppant. The proppant canhave a core and at least one shell surrounding or encapsulating thecore. The core can comprise, consist essentially of, or consist of oneor more ceramic materials and/or oxides. The shell can comprise, consistessentially of, or consist of at least one ceramic material and/oroxide. The examples of various ceramic materials or oxides thereofprovided above can be used here in this proppant. The sintered proppantcan have a core strength to shell strength ratio of from 0.8 to 1. As anoption, the proppant can have an overall proppant strength to corestrength ratio of 2 to 3. The reference to core strength is based on thestrength measurement of the core alone without any shell, for instance,as tested in a crush strength measurement, for instance, according toAPI Recommended Practice 60 (RP-60). The shell strength is determined bydiameteral splitting tensile strength test method based on ASTM C1144,Modulus of Rupture test based on ASTM C78, or Modulus of Rupture testbased on ASTM C1609. Similarly, the overall proppant strength is basedon the proppant with the core and shell tested for crush strengthcompared to the core strength alone. In the present invention, as anoption, the core strength is equal to the shell strength, and can bebelow (lower than) the shell strength, and can be significantly below.The shell can be formed by a plurality of particles which are formed asa ceramic coating around or encapsulating the core and sintered to forma sintered continuous shell.

For purposes of the present invention, the plurality of green and/orsintered ceramic proppants having a monodispersed size means that theproduction of the proppants from a process produces monodispersedproppants without the need for any classification. Also, a plurality ofgreen and/or sintered ceramic proppants having a monodisperseddistribution that is at least a 3-sigma distribution means that theplurality of green and/or sintered ceramic proppants is not achievableby standard air classification or sieving classification techniques. The“plurality,” for purposes of the present invention, refers to at least 1kilogram of proppant, such as at least 5 kilograms, at least 10kilograms, at least 50 kilograms, or at least 100 kilograms of proppant,which would have this monodispersity of the present invention.

With regard to the plurality of sintered ceramic proppants, it isunderstood that the sintered ceramic proppants are preferablysynthetically prepared. In other words, all components of the proppantsare formed by processing into a desired green body shape that isultimately sintered. Put another way, the sintered proppants of thepresent invention preferably do not have any naturally preformed spherespresent (e.g., no preformed cenospheres), unless it is ground toparticle sizes for use in forming the green body. Thus, the sinteredceramic proppants of the present invention can be considered to besynthetically formed.

With the ceramic proppants of the present invention, various propertyimprovements can be achieved. For instance, the crush strength/weightrelationship or ratio is significantly improved. With the presentinvention, for the same size proppant, the proppants can achieve ahigher crush strength (PSI) and, at the same time, permit more porosityin the proppant, which can be beneficial to lowering the specificgravity or density of the proppant. Porosity in a proppant is considereda flaw by those in the proppant industry and ceramic industry. However,the existence of pores or voids is important because even though thesepores or voids are considered flaws, they permit the proppant to have adesirable lower specific gravity or density. However, there is atrade-off in that with porosity in the proppant, this leads to proppantfailure due to affecting the overall crush strength of the proppant.Thus, there is a desired balance between crush strength and porosity. Inprevious proppants, this balance meant that the crush strength of aconventional proppant was lower than desired and, in fact, the desiredporosity was lower than desired, since any increase in porosity wouldlead to a lower crush strength and a proppant that would be considerednot desirable due to low crush strength. With the present invention,high crush strength in combination with high porosity can be achievedand this can be achieved by managing the flaw (pore or void) size, theflaw population, and/or flaw tolerance. One way to better understand theproperty balance achieved with the present invention is to provideseveral examples. For instance, for a ceramic proppant of the presentinvention having a d₅₀ size of 321±24 microns, the crush strength (asdetermined by API RP-60) was 3.73% fines at 20,000 psi, and thisproppant had a total porosity (by volume based on the overall volume ofproppant) of 7.98%. Another example is for a ceramic proppant of thepresent invention having a d₅₀ size of 482±30 microns, the crushstrength (as determined by API RP-60) was 5.08% fines at 20,000 psi, andthis proppant had a total porosity (by volume based on the overallvolume of proppant) of 5.79%. A further way to understand the presentinvention is with respect to the strength/porosity relationship. Thestrength of a proppant (according to API RP-60) is given by thepercentage of fines generated at a given load, say 20,000 psi. Therelationship may be understood by taking the ratio of crush fines to theporosity, i.e. % fines/% porosity to give a dimensionless number whichrepresents the strength/porosity relationship. By doing so with thepresent invention, a strength/porosity descriptor can be establishedwhich, in the present invention can be from 0.4 to 0.9, or from 0.46 to0.88, or from 0.467 to 0.877, such as from 0.5 to 0.8, or from 0.5 to0.85, or from 0.6 to 0.75, or from 0.55 to 0.7, or from 0.55 to 0.8 andthe like.

Another property improvement of the present invention is with regard toachieving a measured specific gravity that is equal or nearly equal(e.g. within 10%, within 7%, within 5%, within 2.5%, within 1%, within0.5%, within 0.25%, or within 0.1%) to the specific gravity calculatedfrom the bulk density of the proppant. The specific gravity is measuredusing the Archimedes method. Generally, with conventional proppants, themeasured specific gravity is higher than the specific gravity calculatedfrom the bulk density of the proppants. For instance, this can be over10% greater. This higher value in measured specific gravity generallymeans that the proppant is “leaking” which means that there are flaws orcracks, or imperfections on the surface of the proppant. With thepresent invention, the “leaking” can be avoided or substantially reducedand this is reflected in the measured specific gravity being the same ornearly equal to the specific gravity calculated from the bulk density.As an option, the bulk density/SG ratio can further include excellentmaximum load strength. The maximum load is determined based on AP 60 andis a determination of the maximum load an individual proppant canwithstand before proppant failure. With the present invention, theproppants (individual proppant) of the present invention can achieve amaximum load (in N) of at least 18 N, such as from 18 to 27 N, or from20 N to 25 N, or from 21 N to 26 N, from 20 N to 100 N, from 30 N to 100N, from 40 N to 100 N, from 20 N to 80 N, from 20 N to 60 N, and thelike. These maximum loads can especially be achieved for proppants thatare spherical (such as ones having an average diameter of from 100microns to 500 microns, or from 150 microns to 450 microns, or from 200microns to 400 microns, or from 250 microns to 350 microns). Theproppant can have a hollow center or have a solid center (e.g., hollowcore or solid core).

Another property that can be achieved with the present invention is anexcellent crush strength with a low coefficient of variance. The realityof proppants is that each single proppant in a plurality of proppantswill not have the exact same crush strength. There is a variance ofcrush strength per proppant. This is due to a number of factorsincluding, but not limited to, the fact that each proppant is notidentical to each other due to size, shape, flaws within the proppant,and the like. Thus, it is highly desirable to have consistent crushstrength per individual proppant in a plurality of proppants(especially, with respect to the lot or batch that goes into a fracturelocation). This consistency can be seen by determining the coefficientof variance with regard to single proppant testing for crush strength.For instance, 30 proppants can be tested individually (30 tests) forindividual crush strength of each proppant in the test group. Then, theaverage crush strength in psi can be determined and then the coefficientof variance can be determined. As one example, in a 30/40 mesh ceramicproppant of the present invention, 30 proppants were individually testedfor crush strength following API test procedure, RP-60, and the averagecrush strength was 31,360 psi. The coefficient of variance was 13.94%,which was based on the standard deviation of 4,371 psi. The d₁₀ of the30 tested proppants was 26,764 psi. This shows a very good lowcoefficient of variance, meaning the weakest proppants in the 30 thatwere tested were relatively close to the average crush strength of the30 spheres, thus showing a low variance in crush strength for theplurality of proppants. This is different fromconventional/commercially-available ceramic proppants, which have acoefficient of variance from the average crush strength of over 25%,such as from 25% to 40%. This is a significantly larger variance incrush strength. Thus, a property of the present invention is that aplurality of proppants (such as 30 spheres or 50 spheres or 100 spheres,or 1 kilogram of proppants) have an average crush strength in psi asdetermined per single proppant and the coefficient of variance of theproppants for individual crush strength can be 20% or less, such as from5% to 20%, or from 5% to 15%, or from 5% to 10%, or from 10% to 20%,with regard to the coefficient of variance.

The proppants of the present invention can also have a low coefficientof variance with regard to size and shape for a plurality of proppants.For instance, the proppants of the present invention can have acoefficient of variance for size (size CV) of 10% or less, and the sameplurality of proppants can have a coefficient of variance for the shape(shape CV) of 5% or less. The shape CV is typically for a sphere. Thecoefficient of variance for size is as described earlier. The shape CVis determined in a manner similar to that of the size CV, that is anumber of particles, say 100, are measured to determine the sphericityand roundness. Based on these individual measurements a mean andstandard deviation value for both the sphericity and roundness can bedetermined. With these values and a modification of the previousexpression for the size CV, two distinct values for the shape CV may beobtained, i.e. the coefficient of variance for roundness(CV_(Roundness)) and the coefficient of variance for the sphericity(CV_(Sphericity)). The roundness CV can be determined from the followingexpression;

${CV}_{Roundess} = \frac{\sigma_{Roundness}}{\mu_{Roundness}}$${CV}_{Sphericity} = \frac{\sigma_{Sphericity}}{\mu_{Sphericity}}$

where σ_(Roundness) and σ_(Sphericity) are the standard deviations forroundness and sphericity respectively, and μ_(Roundness) andμ_(Sphericity) are the mean values for the roundness and sphericityrespectively. For example, the size CV can be 10% or less, such as from1% to 10%, 1% to 8%, 1% to 7%, 1% to 6%, 1% to 5%, from about 3% to 10%,or from about 3% to 8%, or from about 3% to 7%, and the like. The shapeCV can be 5% or less, such as from 0.5% to 5%, or from 0.5% to 3%, orfrom 0.5% to 2%, and the like. Previously conventional ceramic proppantsdid not achieve a size CV and a shape CV as shown herein, thus showingthe abilities of the present invention with regard to achieving highlyuniform proppants with regard to size and shape or sphericity. Withregard to this test, the size CV and shape CV are based on testing atleast 100 individual proppants, such as at least 500 individualproppants, or at least 1 kilogram of proppants, or at least 5 kilogramsof proppants, or at least 10 kilograms of proppants.

The present invention further relates to obtaining synthetic templates(or cores) which can serve as a template to receive one or more shelllayers or can be used by itself. In the present invention, the synthetictemplates of the present invention can achieve very low fines whencrushed at 20,000 psi. For instance, the 20,000 psi crush fines canaverage 5.5% (by weight of total templates) or less (e.g., 5% or less,4% or less, 3% or less, 0.5% to 5.5%, 1% to 5%, and the like). The % canbe consider weight % based on the total weight of material subjected tothe crush test under API RP-60 or similar test. This 5.5% or less crushfines is especially applicable when the sintered d₅₀ size of thesynthetic template is 500 microns or less, such as from 500 microns to100 microns, or 475 microns to 200 microns, or 475 microns to 300microns. This is also especially applicable when the specific gravity ofthe sintered synthetic template is 3 sg or lower, such as 2.9 sg to 2sg, or 2.9 sg to 2.5 sg. The reference to “template” can be considered a“core” here and throughout the present application.

As an option, in the present invention, the present invention achieves acrush resistance #(number) based on the overall crush fine ratio that isdetermined as follows:

Crush resistance Number (CR)={[D×Sd ₅₀ ]/[CF×P]}×10⁶

In the above formula, CF represents the amount (by weight % in fraction)of the crushed fines from a 20,000 psi crush test and is an average.This crush test is based on API RP-60. The weight percent is based onthe total amount of particles being subjected to the crush test. D isdensity of the proppant being tested and is in

$\frac{g}{{cm}\; 3}.$

Sd₅₀ represents the sintered d₅₀ size of the particles being tested inmicrons (μm) and P is crush fine measurement pressure in g/cm² (withpsi=70.3 g/cm²), which is 20,000 psi for this test.

In the present invention, an excellent balance with regard to specificgravity, size, and the low crush fines can be achieved, and, in theabove formula, this is represented by a crush resistance number of fromabout 0.5 to about 3, or from 0.5 to 3, from 0.75 to 2.75, from 1 to2.5, from 1 to 2, from 0.7 to 1.9, and the like. Some specific examplesfrom proppants of the present invention is set forth below. Each ofthese proppants had a core/shell design and where made in a similarmatter to Example 1 in the Examples section. Though not part of theactual CR number, testing at a 25,000 psi crush strength is alsoprovided.

Sample Sd₅₀, Crush fines, % Crush Resistance No. D, g/cm³ μm 20 ksi 25ksi 20 ksi 25 ksi 1 2.80 325 ± 23 4.31 7.37 1.50 0.70 2 2.89 321 ± 243.54 6.26 1.86 0.84 3 2.89 320 ± 28 4.35 7.78 1.51 0.68 4 2.84 475 ± 285.63 8.22 1.70 0.93 5 2.97 482 ± 30 5.43 9.39 1.87 0.87 6 2.95 475 ± 355.82 10.13 1.71 0.79

Further, as an option, the present invention can achieve, with regard tothe synthetic template (or core) (such as a ceramic core or template) orthe overall proppant, an excellent strength to porosity ratio which canbe determined by measuring the crush strength of the proppant ortemplate and dividing by the amount of porosity (including any centralvoid) that is present in the proppant. For instance, in the presentinvention, the proppant or template of the present invention can achievea strength (psi)/porosity (percent volume based on total volume ofmeasured particle) of from 5×10⁴ to 150×10⁴, such as from 5×10⁴ to40×10⁴, or 10×10⁴ to 30×10⁴, or 15×10⁴ to 30×10⁴, or 5×10⁴ to 10×10⁴.

The proppants of the present invention can be made as follows. A slurrycontaining green particles (e.g., milled particles) can be prepared,which ultimately is fed into a spray dryer. The materials that form thegreen body can be considered the green body material that is a mixtureand is formed into a slurry of green body material. The spray dryer,based on the nozzle design, creates green bodies having desired shapes.For instance, the green bodies can have a highly spherical shape androundness. The diameter of the green bodies can typically be from about10 microns to about 1,000 microns, such as from about 20 microns toabout 250 microns. In making the slurry containing the green particles,the particles are generally a mixture of two or more ceramic and/orceramic precursor materials. The green particles that are in the slurrycan have a particle size of from about 0.3 micron to about 50 microns,such as from about 0.5 micron to about 5 microns. The green particlesthat are present in the slurry that ultimately form the green body canbe initially prepared by taking the raw materials that form the greenbody, namely ceramic and/or ceramic precursors, and reducing the size ofthe material to the desired diameter, such as by attritor milling orother milling techniques.

As an option, in the present invention, the green body, for instance,that can form a template or core, can be solid throughout the greenbody. In other words, as an option, there is no void, including nocenter void. Put another way, the green body is not a hollow green body.With the present invention, even though the green body can be a solidthroughout the green body, the resulting proppant which is a sinteredproppant, can result in having a void in the center of the sinteredproppant. In other words, the sintered proppant can be hollow in thecenter. This can occur when the green body is comprised of a solid coreand at least one shell-forming material forms a shell around the core.The green body that comprises the green body core and green body shellcan be sintered, and, during sintering, part or all of the core diffusesto or within the shell, such as in a very systematic way. This resultsin forming a hollow portion or void in the proppant generally in thelocation of the core (e.g., geometrical center of proppant sphere). FIG.5 and FIG. 6 are SEM figures. FIG. 5 shows a cut-opened 40/50 proppantmade from synthetic materials, and FIG. 6 shows a cut-opened 30/40proppant made from synthetic materials, wherein at the center of eachSEM, a hollow void can be seen which was formed during sintering, butdid not exist prior to sintering. This hollow void generally can be theshape and size of the original green body or a portion thereof thatformed the core (e.g., from 10% to 100% of the green core, or 20% to80%, or 30% to 70%, or 40% to 60% by volume of the green core). Asstated, at least some of the material that formed the core of the greenbody diffused into the shell which surrounds this hollow space. Thisdiffusion provides a mechanism for strengthening the shell, as well asthe overall proppant.

Test methods for determining the magnitude of residual strain within thematrix. The residual strain due to thermal mismatch caused by thediffusion of the template material into the shell matrix can bedetermined by collecting the electron diffraction pattern of a specificcrystalline phase present in the matrix during transmission electronmicroscopy (TEM) analysis. The presence of strain within the crystallinephase, and consequently the matrix will manifest itself as a deviationin the electron diffraction pattern shape and spot positions from theunstrained condition. The magnitude of the deviation from the unstrainedcase would allow calculation of the magnitude of the residual strainresponsible for such shifts in the electron diffraction pattern.

Another method to determine the presence of residual strain is throughthe use of nano-indentation. In the case of an unstrained material, thedimensions of the indentation impression and any radial cracks formed atthe verticies of the indentation site are solely dependent upon thematerial properties. The presence of a residual strain in the matrixwould lead to a change in both the indentation impression dimensions andthe dimensions of the radial cracks. In the case of the residual straincomponent being compressive, the indentation impression dimensions wouldbe smaller than the unstrained case and the resulting radial cracks (ifany) would be much shorter than the unstrained case. In the case of atensile residual strain being present, the indentation impressiondimensions would be larger and the radial cracks would be longer thanthe unstrained case.

As shown in FIG. 8, based on the schematic or diagram shown, a sinteredproppant with a central void (90) is shown. The sintered proppant has ageometrical center within the sphere (110), and the central void (108)can be located in the center part of the sphere which is where part orall of the green core was located prior to diffusing into the shell(95). More specifically, the interface between the hollow void formed inthe shell interface is shown as (106). The region from the area startingat about 104 to the interface 106 can be representative of where amajority (by weight) of the green core diffuses (over 50 wt % of thediffused material) into the shell area. Area 102 in FIG. 8 isrepresentative of where very little or no core material diffuses (e.g.,less than 25 wt % of the diffused material) into the shell and canconsist of the shell material only in a sintered state. 100 is thesurface of the proppant. As shown in the three graphs that are part ofFIG. 8, which are in alignment with the proppant diagram, one can seethat the porosity, of course, is highest in the central void area andthat is due to the diffusion of part or all of the green core into theshell regions. Initially, the porosity from the void-solid interface(106) to area 104 (the circumference of 104), the porosity is lowbecause the diffusion of the core material fills the pores (if any) inthe circumferential region between 106 and 104. Then, in thecircumferential area from 104 to 102, the porosity is higher(approximately 1% to 20% higher by volume) than region 104 to 106because the porosity in this area has not been filled or notsubstantially filled with any diffused core material. Then, thecircumferential area from 102 to 100 (the surface of the proppant) hasvery little or no porosity (e.g., from 0% to 5% by volume in this area)because a higher temperature is typically reached in this area duringsintering and this removes or closes all or most of the pores at thisnear surface region. Thus, as an example, the proppant of the presentinvention can have a central void with porosity that is highest in thecentral location of the shell with regard to radius of the sphere. Morespecifically, the region from A to B shown in FIG. 8 has from 0% to 5%(by volume) of porosity, such as from 0% to 1% by volume porosity. Theregion from B to C has porosity on the order of from 5% to 30% by volumeof that region, more specifically from 10% to 20% by volume in thatregion, and the region from C to D has porosity that is the same orabout the same as the porosity from region A to B (±10%). The regionfrom A to B can be considered the first region; the region from B to Ccan be considered the second region or middle region of the shell; andthe region from C to D can be considered the third region or outerregion of the shell. The second region has more porosity by volume thanthe first region and/or the third region. The second region can haveporosity that is from 10% to over 100% more compared to region 1 orregion 3. The first region can comprise from 10% to 40% by volume of theoverall non-void region of the proppant, such as from 10% to 30% byvolume. Region 2 can comprise from 20% to 50% by volume of the overallnon-void regions of the proppant and region 3 can comprise from 10% to40% by volume of the overall non-void regions of the proppant.

The second graph shown in FIG. 8 provides a showing of the diffusion ofthe core concentration which can be, for instance, crushed and/or milledcenospheres. As can be seen in the graph, the void would represent anarea where no core concentration remains since it diffused into theshell. The diffusion of the core material is represented by plotting theconcentration (as measured by energy dispersive spectroscopy) of one ofthe elements contained in the core material (for example, iron, ifpresent). The concentration profile is not linear but rather follows apower law which decreases from the interior regions to the exteriorregions of the proppant. The highest remnants or migration of the coreis where core diffusion occurred at circumferential region A to B. Fromcircumferential regions B to C and C to D, the amount of core diffusioncan gradually decrease in a linear or somewhat linear manner. The coreconcentration in the first region can be the highest (by weight),wherein the third region (C to D) can be the lowest with regard todiffusion amount of the core material. In comparing the first regionwith the second region and the third region, with regard to the amountof core which is diffused in these three regions, the first region canhave 3× to 5× (by weight) more diffused core material than the secondregion and 10× to 20× (by weight) more than the third region. The thirdgraph shown in FIG. 8 shows the formation of whiskers in situ. Thewhisker concentration can mimic the core diffusion concentration in thefirst region, second region, and third region. Therefore, for purposesof the present invention, the concentration levels of the whiskers canbe identical or nearly identical (±10%) to the core concentrationsdescribed above and apply equally to this description of whiskerconcentrations.

For purposes of the present invention, with regard to the green bodycore, from about 50% to about 70% by weight of the overall green bodycore can diffuse into the shell, such as from 60% to 90%, 70% to 90%,80% to 90%, all based on the weight of the green body core.

As a more specific example, the green body core can comprise or bemilled cenospheres and/or fly ash, which can optionally contain binderto form the green body. The green body shell material can comprisealumina, optionally with other ceramic materials or oxides. Thediffusion of the core into the shell (at least partially) is or can bedue to the glassy ingredients or nature of the green body core,especially when the core is or contains a cenosphere or fly ash or bothor at least comprises ground cenospheres and/or fly ash. This migrationor diffusion of the green body core into the shell can occur via liquidphase infiltration of the ceramic shell matrix by the molten corematerial at or near the sintering temperature of the ceramic shell, thusleading to densification of the ceramic shell by viscous or liquid phasesintering processes. The shell, during sintering, can be an example ofsolid state sintering, which ultimately forms a solidified shell.

Generally, the sintering used to achieve this viscous sintering of thecore and the solid state sintering of the shell can be from about 1,000°C. to about 1,600° C. for 10 minutes to 2 hours or more, such as fromabout 1,200° C. to 1,300° C. for 1 to 2 hours, though other times andtemperatures can be used to achieve these effects.

Referring to FIG. 8 and to FIGS. 13-16, FIG. 13 shows a portion of thesintered proppant, wherein the area A signifies the interface areabetween the void and the non-void area indicated by 106 in FIG. 8. AreasB and C shown in FIG. 13 represent regions B to C and C to D in FIG. 8.FIGS. 14, 15, and 16 are enlarged, more magnified versions of each ofthese three areas, respectively. As can be seen in FIG. 14, whiskerconcentration (e.g., mullite whiskers) is shown which, going from thebottom of FIG. 14 to the top of FIG. 14, shows the concentration ofwhisker formation decreasing which would be representative of thewhisker concentration decreasing as shown in the third graph in FIG. 8.In other words, the mullite whisker concentration is decreasing goingfrom region A to C in FIG. 8. Further, FIG. 15, which is an enlargedarea of area B as shown in FIG. 13, shows a higher degree of porositycompared to FIG. 16, which is the near outer surface region of theproppant.

FIGS. 17-20 show the progression of the green body core duringsintering, namely, viscous sintering, which leads to the diffusion of atleast part of the core into the shell. As shown in FIG. 17, oncesintering begins, one can still see the solid core material that formsthe core or template of the green body. At this point, the sinteringstarts and the matrix of the core sinters very slightly and, at thispoint, there is no radial diffusion of the core material. FIG. 18 showsa subsequent picture where the template or core material has begun tomelt and the shell or matrix surrounding the core is sintering slightly,but radial diffusion of the core material or template has not becomesignificant yet. FIG. 19 is a subsequent SEM showing that outward radialdiffusion of the core material has begun and that the shell or matrix issintering further and a hollow core is being formed during to thediffusion of the core or template material into the shell regions. FIG.20 finally shows further outward radial diffusion of the core materialinto the shell. FIG. 19 shows a radial diffusion of about 15 microns,where the diffusion depth is now about 25 microns in FIG. 20. Further,as shown in FIG. 20, the formation of a void or hollow core is occurringand the shell or matrix, at this point, is sintering substantially bysolid state sintering.

The benefits of the present invention can be seen by taking SEM imagesof the proppants or templates of the present invention and comparingthem to conventional cenospheres. For instance, FIGS. 21 and 22represent typical conventional cenospheres that have previously beenused in the formation of proppants. As can be seen in FIGS. 21 and 22,conventional pre-formed cenospheres have irregularities, surfacedefects, and structural defects as especially seen in the cross-sectionof one of these conventional cenospheres set forth in FIG. 22. Unlikethe conventional cenospheres, in the synthetic templates of the presentinvention, high regularity and uniformity and low defects are achievedwith the present invention. As can be seen in FIG. 23, a sinteredsynthetic template of the present invention, which was formed byspray-drying techniques, clearly shows the high uniformity and very lowsurface defects that exist on a synthetic template particle of thepresent invention. FIG. 24 a is a cross-section of one of these sinteredsynthetic templates which is a solid and, again, as can be seen incomparing FIG. 24 a with FIG. 22, the present invention achieves a verydifferent morphology and very low defects. FIG. 24 b shows the samesolid synthetic template after being sintered in air at 1000° C. FIG. 25further shows a sintered hollow synthetic template of the presentinvention and, again, as can be seen, low surface defects are achievedand the surface is quite uniform especially compared to the conventionaltemplates.

The present invention relates, in part, to a method of forming a ceramicproppant having a ceramic core and ceramic shell structure. The methodinvolves forming a solid green body core and forming a green shell(s)around the core, wherein the shell comprises one or more ceramicmaterials. The shell can be considered a ceramic shell. The method theninvolves sintering the green body that comprises the core and shell(s)such that at least part (or all) of the ceramic material that definesthe core diffuses into the shell to result in a ceramic proppant havinga center void (or hollow core) and a ceramic shell.

The partial or complete diffusing of the core into the shell occursduring sintering, and the diffusing can be uniform such that a portionor the entire core diffuses uniformly throughout the shell regions orthe diffusing can be in a gradient fashion such that a higherconcentration of the core that diffuses into the shell is located closerto the core than to the exterior outer surface of the proppant.

In this method, the green body shell has an overall higher sinteringtemperature than the green body core. Put another way, the softeningtemperature of the green body shell is higher than the softeningtemperature of the green body core. For instance, the softeningtemperature of the green body shell is at least 100° C. higher than thesoftening temperature of the green body core and, more preferably, is atleast 200° C. higher, such as from 200° C. to 400° C. higher compared tothe softening temperature of the green body core. As an example, thesoftening temperature of the green body shell is from about 300° C. toabout 400° C. higher than the softening temperature of the green bodycore. “Softening temperature” is the average softening temperature. Thegreen body shell can be porous (e.g., uniformly or non-uniformly) and ispreferably porous. The porosity can be non-interconnecting. In otherwords, the pores are not connected or bridged in any manner. Forinstance, the green body shell has a porosity (before sintering) of atleast 10%, at least 20%, at least 30% by volume based on the overallvolume of the green body shell, such as from 10% to 40% porosity byvolume prior to sintering. After sintering, and after the optionaldiffusion referred to above and described herein, the sintered shell canhave a porosity of 5% by volume or more, such as at least 10% by volume,wherein volume is a reference to the shell volume after sintering. Forinstance, the shell can have a porosity by volume of from 10% to about40% based on the overall volume of the sintered shell. Generally, theporosity in the shell after sintering, compared to pre-sintering,decreases, such as by an amount of 5% to 30% or 10% to 25% by volume.

As an option, whiskers and/or platelets, such as mullite whiskers, canbe present in the core and/or shell. For purposes of the presentinvention, “whiskers” are referred to and this includes whiskers and/orplatelets. These whiskers can be formed in situ during the sinteringprocess that forms the sintered proppant. Particularly, and just as anexample, during the diffusion of the core or portion thereof into theshell, as described above, part of the diffusing process permits one ormore of the ingredients that comprise the core to react and formwhiskers, such as mullite whiskers. The concentration of the whiskerscan be uniform throughout the core and/or shell or it can exist as agradient where a higher concentration of the whiskers exists closer tothe sphere center of the proppant. Put another way, the concentration ofwhiskers can be higher near the core and at the interface between thecore and shell and have a lower concentration (such as at least 20%lower, at least 30% lower, at least 40% lower, at least 50% lower, atleast 60% lower with regard to the weight amount of whiskers present ator near the surface (within 15% of the surface by radius) of theproppant compared to the concentration at the core-shell interface). Theformation of whiskers in situ leads to enhanced strength andreinforcement of the overall proppant.

In the present invention, as an option, one or more nucleating agentscan be used in the green body or part(s) thereof (e.g., core part and/orshell part). The nucleating agents can be TiO₂, Li₂O, BaO, MgO, ZnO,Fe₂O₃, ZrO₂, and the like. The nucleating agents can be present in thegreen body from 0 wt % to 15 wt %, based on the weight of the greenbody, such as from 0.01 wt % to 15 wt %, or 0.1 wt % to 15 wt % or more,or 1 wt % to 10 wt %, or 2 wt % to 5 wt % and the like. The wt %provided here can alternatively apply to a part of the green body, forinstance, to the core part and/or to the shell part of a proppant, if ashell is present. With the use of nucleating agents, the nucleatingagents can promote glass ceramic material generation. For instance,nucleating agents can be used in the green core body material, and agreen shell material can be applied to the green body core and then thenucleating agents in the green core body can diffuse or migrate to theshell and promote glass ceramic generation in the shell. With the use ofnucleating agents, the shell or matrix can have an initial amorphousphase of 0% to 100% and then after sintering, the crystallinity canrange from 100% to 0% from the inner to outer surface of the proppant.With the use of nucleating agents, improved mechanical strength can beachieved and/or improved chemical stability of the proppants.

In the present invention, as an option, one or more anisotropic growthpromoters can be used in the green body. The growth promoters can beadded to the green slurry used to form the green body (such as the greenbody core and/or green body shell). The growth promoters can be one ormore oxides. For instance, several oxides are capable of promotinganisotropic growth of whiskers in ceramic material such as, but notlimited to, alumina, boehmite, alumina precursors (gibbsite, bauxite).The growth promoters are more effective in promoting growth of whiskers,such as mullite whiskers, at temperatures ranging from 1000° C. to 1650°C. These oxides include TiO₂, MnO₂, Cr₂O₃, CaO, K₂SO₄, K₂CO₃, MgO, AlF₃and SrO, and the like. Mixtures of Na₂O—MgO—Al₂O₃ and CaO—SiO₂—Al₂O₃ arealso able to form anisotropic aluminate structures (platelets).Anisotropic grains/precipitates strengthen (or toughen) the matrix bypreventing catastrophic growth of cracks in the matrix. Precipitates orclusters with high aspect ratios create torturous paths for the crackseither by blunting or by diverting/changing directions of the crackpaths. Needle shaped mullites and platelet shape alumina and aluminatesare some examples of the high aspect ratio structures.

In the present invention, for proppants, one can produce spray driedsynthetic template cores (solid or hollow) from ceramic material, suchas alumina, boehmite, gibbsite, and/or particulate mullite, and thelike. One can also introduce anisotropic growth promoters in the shellgreen material during the spray coating of the shell forming greenmaterial onto the templates. During sintering of such green proppants,radial diffusion and migration of core materials would encounteranisotropic growth promoters at high temperatures, and their particulateshape would change to shapes having high aspect ratios (e.g. needle,platelets, laths, and the like). The growth promoters can be used in anamount of from about 0.5 to about 25 wt % based on the overall weightpercent of the green body.

The proppants described herein, of the present invention can include oneor more of the following characteristics:

said glassy phase (or amorphous phase) is present in an amount of atleast 10% by weight, based on the weight of the proppant (e.g., at least15%, at least 20%, at least 25%, at least 30%, at least 40%, at least50%, such as from 15% to 70%, all based on wt %, based on the weight ofthe proppant);

said ceramic whiskers have an average length of less than 5 microns(e.g., less than 4 microns, less than 3.5 microns, less than 3.2microns, less than 3 microns, less than 2.7 microns, less than 2.5microns, less than 2.2 microns, such as from 0.5 micron to 5 microns, orfrom 1 micron to 3.5 microns, or from 0.8 micron to 3.2 microns, or from1 micron to 3 microns or from 1.2 to 1.8 microns);

said ceramic whisker have an average width of less than 0.35 micron(e.g., less than 0.3, less than 0.28, less than 0.25, less than 0.2,less than 0.15, such as from 0.05 to 0.34 micron, from 0.2 to 0.33micron, from 0.1 to 0.3 micron, from 0.12 to 0.2 micron, all units inmicrons);

said ceramic whiskers have a whisker length distribution, d_(as), ofabout 8 or less (e.g., 7 or less, 6 or less, 5 or less, 4 or less, 3 orless, 2 or less, 1 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2or less, such as 0.1 to 8, 0.1 to 7, 0.1 to 6, 0.1 to 5, 0.1 to 4, 0.1to 3, 0.1 to 2, 0.1 to 1, 0.1 to 0.75, 0.1 to 0.5, 0.1 to 0.3, 0.1 to0.2, 0.1 to 1.8), wherein, d_(as)={(d_(a90)−d_(a10))/d_(a50)} whereind_(a10) is a whisker length wherein 10% of the whiskers have a smallerlength, d_(a50) is a median whisker length wherein 50% of the whiskershave a smaller whisker length, and d_(a90) is a whisker length wherein90% of the whiskers have a smaller whisker length;

said proppant having an free alpha-alumina content of at least 5% byweight of said proppant (e.g., 5 wt % to 50 wt % or more, at least 10 wt%, at least 20 wt %, at least 30 wt %, at least 40 wt %, based on theweight of the proppant);

said proppant having an HF etching weight loss of less than 35% byweight of said proppant (e.g., less than 30% by weight, less than 25% byweight, less than 20% by weight, less than 15% by weight, less than 10%by weight, such as from 10 wt % to 34 wt %, from 15 wt % to 30 wt %,from 18 wt % to 28 wt % by weight of said proppant);

said proppant has a major phase of whiskers of less than one micron anda secondary minor phase of whiskers of one micron or higher; and/or

said ceramic whiskers have a whisker length distribution having d_(a90),which is a whisker length wherein 90% of the whiskers have a smallerwhisker length, of less than 12 microns (e.g., less than 10 microns,less than 8 microns, less than 7 microns, less than 6 microns, less than5 microns, less than 4 microns, less than 3 microns, less than 2microns, such as from 1 to 10, 1.5 to 5, 1.7 to 5, 1.8 to 4, 1.9 to 3.5,1.5 to 3.5).

It is to be understood that all averages and distributions mentionedabove are based on measuring at least 50 whiskers picked on a randombasis in a proppant. Preferably, at least 10 proppants are measured inthis manner and an average obtained.

In the methods of the present invention, the green body can be made fromone or more ceramic or ceramic precursor particles, and can comprise,consist essentially of, or consists of cordierite, mullite, bauxite,silica, spodumene, silicon oxide, aluminum oxide, sodium oxide,potassium oxide, calcium oxide, zirconium oxide, lithium oxide, ironoxide, spinel, steatite, a silicate, a substituted alumino silicateclay, an inorganic nitride, an inorganic carbide, a non-oxide ceramic orany combination thereof. The green body material can be or include oneor more sedimentary materials (e.g., feldspar, quartz, amphiboles, clay,shale, siltstone, sandstone, conglomerates, breccias, quartz sandstone,arkose, greywacke, quartz arenites, lithic sandstone or any combinationsthereof) and/or synthetically produced materials (e.g., milledcenospheres). As an option, the green body material is not igneous ormetamorphic materials and/or the resulting proppant of the presentinvention can have the complete absence or substantial absence (e.g.less than 1% by weight of proppant) of igneous or metamorphic materials,which can be less suitable for certain proppant uses.

The particles that form the green body can have any particle sizedistribution. For instance, the particles that form the green body canhave a particle size distribution, d_(gs), from about 0.5 to about 15,wherein, d_(gs)={(d_(g90)−d_(g10))/d_(g50)} wherein d_(g10) is aparticle size wherein 10% of the particles have a smaller particle size,d_(g50) is a median particle size wherein 50% of the particles have asmaller particle size, and d_(g90) is a particle size wherein 90% of theparticle volume has a smaller particle size. The particle sizedistribution, d_(gs) can be from 0.5 to 15, from 0.75 to 12, from 1 to6, from 1 to 10, from 1.5 to 8, from 2 to 8, from 2.5 to 8, from 2.5 to6, from 3 to 10, from 1 to 8, from 0.5 to 10, from 0.5 to 1, from 0.5 to2, from 0.5 to 3, from 0.5 to 4, from 0.5 to 5, from 0.5 to 6, from 0.5to 7, from 0.5 to 8 or any various combination of ranges providedherein.

The median particle size, d_(g50), of the particles that form the greenbody can be of any median size, for instance, from about 0.01 μm toabout 100 μm, wherein d_(g50) is a median particle size where 50% of theparticles of the distribution have a smaller particle size. The medianparticle size, d_(g50), of the particles that form the green body can befrom about 1 μm to about 5 μm, from about 1 μm to 2 μm, from 0.01 μm to100 μm, from 0.05 μm to 100 μm, from 0.1 μm to 100 μm, from 0.5 μm to100 μm, from 0.75 μm to 100 μm, from 1 μm to 100 μm, from 2 μm to 100μm, from 5 μm to 100 μm, from 10 μm to 100 μm, from 20 μm to 100 μm,from 0.01 μm to 10 μm, from 0.05 μm to 10 μm, from 0.1 μm to 10 μm, from0.5 μm to 10 μm, from 0.75 μm to 10 μm, from 1 μm to 10 μm, from 2 μm to10 μm, from 5 μm to 10 μm, from 0.01 μm to 5 μm, from 0.05 μm to 5 μm,from 0.1 μm to 5 μm, from 0.2 μm to 5 μm, from 0.3 μm to 5 μm, from 0.4μm to 5 μm, from 0.5 μm to 5 μm, from 0.75 to 5 μm, from 2 μm to 8 μm,from 2 μm to 6 μm, from 1 μm to 20 μm, from 1 μm to 30 μm, or anyvarious combination of ranges provided herein, wherein d_(g50) is amedian particle size where 50% of the particles of the distribution havea smaller particle size.

The particles that form the green body or a portion of the green body,such as the green body core or green body shell, can have a unimodalparticle size distribution or it can be a multi-modal particle sizedistribution, such as a bi-modal particle size distribution. Forexample, as one option, the green body core can be formed from aunimodal or bi-modal or other multi-modal particle size distribution. Asa preferred option, the core can be formed from a bi-modal particle sizedistribution which results in a tighter particle backing, and the greenbody shell, if used, can be formed, as a preference, with a unimodalparticle size distribution which results in less packing density andtherefore permits diffusion of the green body core (as described aboveas an option) into the shell area or radial portion thereof. Thus, inthe present invention, a proppant can be formed comprising a pluralityof micron particles that are sintered together, wherein the micronparticles have a unimodal particle distribution or it can have abi-modal particle distribution. The micron particles can have a d₅₀ of0.5 micron to 3.5 microns. The green body and/or resulting proppant canhave a plurality of pores having a pore volume wherein the majority ofthe pore volume results from the interstitial gaps formed between themicron particles. The pore volume created in this manner can be fromabout 1% to 30%, or from about 5% to about 20%, based on the totalvolume of the proppant either in the green state or sintered state. Thed₁₀ of the micron particles used to form the green body can be within100% of the d₅₀, or within 50% of the d₅₀. The micron particles used toform the green body can have a d₉₀ that is within 100% of the d₅₀ orthat is within 50% of the d₅₀. Further, micron particles used to formthe green body can have a d₁₀ that is within 100% of the d₅₀ and have ad₉₀ that is within 100% of the d₅₀ or can have a d₁₀ that is within 50%of the d₅₀ and have a d₉₀ that is within 50% of the d₅₀. As stated, thecore and/or shell can comprise a plurality of micron particles that havea d₅₀ of from 0.5 micron to 3.5 microns and are sintered together,wherein the micron particles have a bi-modal particle distribution witha Modal A particle distribution and a Modal B particle distribution. Themicron particles of each modal (A and B) can have a d₅₀ of 0.5 micron to3.5 microns, and Modal A can have a d₅₀ that is at least 10% differentfrom the d₅₀ from Modal B or at least 20% different from the d₅₀ ofModal B, or Modal A can have a d₅₀ that is from 10% to 100% differentfrom the d₅₀ of Modal B.

With a tri-modal particle size distribution that forms the green body ora portion thereof, such as the core or shell, reduced porosity can beachieved and enhanced sintering can be achieved.

In the present invention, the green body or a portion thereof, such asthe core or shell, can have a density, as measured by a gas pycnometer,such that the average density (g/cm³) does not alter by more than 1%between the density of the whole green body compared to the density ofthe crushed green body, and preferably the average density is the samefor the whole green body compared to the crushed green body. In otherwords, the average density changes 0% or 0.005% or less. Put anotherway, the average density of the green body or a portion thereof, such asthe core or shell, can be 100%.

As an option, one or more mobile phases can be created in the dropletsof the slurry that forms the green body, such as two phases, and onephase can migrate to the surface of the droplet, which can cause amulti-phase droplet (based on density) to form. This can cause anon-uniform green body of phases which can then cause a difference indiffusion into the shell as described herein. The difference indensities can be at least 10%, at least 20%, at least 50%, at least 100%with regard to the multi-phase droplet that results in the green body.

With regard to the diffusion of at least a portion of the green bodycore into the shell, a higher crystalline content will diffuse slowerthan a semi-crystalline or glassy green body core. Further, the largestamount of diffusion can occur when fine particles of a glassy nature areused to form the green body core, and the green body shell is formedfrom coarse particles of a crystalline nature. Thus, as an option, thegreen body core can contain at least 50% by weight of a glassy materialor at least 75% by weight or at least 95% by weight based on the weightof the green body core and/or the green body shell can contain at least50% of a crystalline material, such as at least 75% or at least 95% byweight based on the weight of the green body shell. Further, theparticles used to form the green body core can be at least 10%, at least25%, at least 50%, at least 100% smaller in the average mean size (d₅₀size) compared to the mean particle size (d₅₀ size) of the particlesthat form the green body shell.

As an option and taking into account that proppant sizes can be relevantto the standard deviations, set forth below are preferred standarddeviation ranges based on mean particle size of the proppant (green orsintered state). For instance, when the mean particle size is from100-299 μm, the standard deviation can be from 0.83 to 2.5. The meanparticle size is a reference to the green body and/or resulting sinteredbody, and the green body can be a template or a template with ashell(s), and/or the resulting sintered version thereof. The rangesprovided for mean particle size and standard deviation can be exactranges or can “about” these ranges (e.g., from about 100 microns toabout 299 microns, or a standard deviation of from about 0.83 to about2.5, and so on).

-   -   100-299 μm, σ=0.83-2.5    -   300-499 μm, σ=2.5-4.16    -   500-799 μm, σ=4.16-6.66    -   800-999 μm, σ=6.66-8.33    -   1000-1499 μm, σ=8.33-12.5    -   1500-2000 μm, σ=12.5-16.66

Based on the particle size distribution to achieve a monodispersedistribution (as specified previously), the diameters of the particlescan fall within a 5% tolerance band about the mean particle diameter:

d _(s)=μ±0.025μ

and d_(s) can be defined by:

$d_{s} = \frac{\left( {d_{90} - d_{10}} \right)}{d_{50}}$

where d₉₀, d₅₀ and d₁₀ are the 90^(th), 50^(th), and 10^(th) percentilesof the particle size distribution respectively. For example, d₉₀ refersto the particle size below which 90% of the particles are below thisparticle size, similarly for the d₅₀ and d₁₀.

Specifying the total particle size distribution width to be less than orequal to 5% of the mean particle size, the following range for d_(s)

0.00<d _(s)<0.05

is obtained.

In the present invention, the ceramic or ceramic precursor can bepresent in the green body in various amounts, such as from about 50% byweight to 100% or to about 99.9% by weight of the green body, from 65%to 99.9%, from 70% to 99.5%, from 75% to 99%, from 80% to 98%, from 85%to 97%, from 75% to 95%, from 80% to 90%, from about 90% to about 99.9%,or any various combination of ranges provided herein, wherein the % is aweight percent based on the weight of the green body.

In order for the slurry to be spray dried, the rheology is preferred tobe in a certain range to obtain desired properties. The sprayability ofslurry is related to and affected by the density, viscosity, and surfacetension of the slurry. These variables are, in turn, affected bychemical composition, solid content, particle size distribution, typeand amount of additives such as binder, dispersant, surfactant and pHand zeta potential (surface charge), and the like. For stable anduniform drop formation during spray drying processes, slurrycharacteristics have an important role. Viscosity, surface tension anddensity determine the balance of viscous, inertial and surface tensionforces during drop formation. A dimensionless characteristic, Z,describing this balance, called the Ohnesorge Number or Z number can beused as a measure of sprayability

$Z = {\frac{\sqrt{We}}{Re} = \frac{\eta}{\sqrt{\rho \; \sigma \; l}}}$

where R_(e) is the Reynold's Number (Re=ρυl/η), W_(e) the Weber Number,(We=ρυ²l/σ), σ the surface tension in N/m, ρ the density of slurry inkg/m³, l the characteristic length (usually the orifice diameter) in m,η the viscosity in Pa·s, and υ velocity in m/s. The range of Z forpreferred spherical drop ejection in spray drying should be in a certainrange, for example from 1 to 10, such as, from 2 to 9, or from 3 to 8,or from 4 to 6. As shown in one set of examples, when the Z number isabove 1 and below 10, slurries had excellent sprayability for spraydrying based on observed results. However, when the Z number was below1, the slurries had poor or less than desirable sprayability which hadto be addressed and/or modified in order to obtain desirable properties.These results are set forth in Example 3.

The green body material can further comprise additional components usedto contribute one or more properties to the proppant or part thereof.For instance, the green body (e.g., the core and/or shell) can furthercomprise at least one sintering aid, glassy phase formation agent, graingrowth inhibitor, ceramic strengthening agent, crystallization controlagent, glass-ceramic crystallization agents, and/or phase formationcontrol agent, or any combination thereof. The sintering promoter can beor include a compound containing zirconium, iron, magnesium, alumina,bismuth, lanthanum, silicon, calcium, cerium, yttrium, a silicate, aborate or any combination thereof. It is to be understood that more thanone of any one of these components can be present and any combinationcan be present. For instance, two or more sintering aids can be present,and so on. There is no limit to the combination of various agents or thenumber of different agents used. Generally, one or more of theseadditional agents or aids can include the presence of yttrium,zirconium, iron, magnesium, aluminum, alumina, bismuth, lanthanum,silicon, calcium, cerium, one or more silicates, one or more borates, orone or more oxides thereof, or any combination thereof. These particularaids or agents are known to those skilled in the art. For instance, asintering aid will assist in permitting uniform and consistent sinteringof the ceramic material or oxide. A glassy phase formation agent, suchas a silicate, generally enhances sintering by forming a viscous liquidphase upon heating in the sintering process. A grain growth inhibitorwill assist in controlling the overall size of the grain. A ceramicstrengthening agent will provide the ability to strengthen the overallcrush strength. A crystallization control agent will assist in achievingthe desired crystalline phase upon heat treatment such as sintering orcalcining. For instance, a crystallization control agent can assist inensuring that a desirable phase is formed such as an alpha aluminumoxide. A phase formation control agent is the same or similar to acrystallization control agent, but can also include assisting inachieving one or more amorphous phases (in addition to crystallinephases), or combinations thereof. The various aids and/or agents can bepresent in any amount effective to achieve the purposes described above.For instance, the aid and/or agents can be present in an amount of fromabout 0.1% to about 5% by weight of the overall weight of the proppant.The proppant can comprise one or more crystalline phases or one or moreglassy phases or combinations thereof.

The green body core can further comprise such additives and/orcomponents that can react or otherwise interact with the ceramic shellor various components thereof during sintering to promote the formationof residual strain fields (microstrains and/or macrostrains) within thesintered proppant body. These reactions between the active components ofthe core and shell materials have the ability to generate additionalphases which exhibit a different thermal expansion coefficient to thecore and/or shell leading to a residual strain field through thecross-section of the proppant shell. Alternatively, the active componentor components of the core, may interact with, or modify the crystalstructure of the shell material through such processes as atomicsubstitution or filling of vacancies within the crystal structure. Thesemodifications of the crystal structure may lead to the formation oflattice strains and/or thermal mismatch strains within the shell. Theformation of such residual compressive strain fields have the ability tolead to improvements in the apparent fracture toughness and strength ofthe ceramic shell and consequently an improvement in the strength of theproppant. In addition, the formation of residual compressive strainfields within the surface regions of the proppant particle, may improvethe corrosion resistance of the ceramic by increasing the apparentactivation energy of the corrosion reaction. These residual strainfields may be characterized using any one of a number of diffractiontechniques, including x-ray diffraction, neutron diffraction orsynchrotron radiation diffraction. The existence of macrostrains canmanifest themselves as a shift in the diffraction peak positions and themicrostrains (or root mean square strain, rms strain) can manifestthemselves as a broadening of the peak width, i.e. an increase in thehalf-width at full maximum (HWFM) value of the peaks. Alternatively, thediffraction patterns can be collected at varying angles of sample tiltand inclination (with respect to the incident radiation beam) using aEulerian cradle to obtain a set of diffraction patterns that will allowthe extraction of the 3 dimensional strain tensor for the system, whichdescribes the macrostrain and microstrain components of the system. Theabsolute value of the total residual strain in the system may range from0% to 5% or higher, such as from 1% to 3% or from 3% to 5%.

The green body material can include reinforcing particulates. Theparticulates can be used for strength enhancement or density control(reduce or increase density), or both. The particulates can be includedin the composition which forms the green body or part thereof, in anyamount such as from about 1 vol % to 50 vol % or more, for example, from5 vol % to 20 vol % of the overall green body or part thereof. Thereinforcing particulates can be ceramic material (e.g., oxide ornon-oxide), metallic material (e.g., metal elements or alloys), organicmaterial, or mineral-based material or any combination thereof. Ceramicparticulates include, but are not limited to, alumina, zirconia,stabilized zirconia, mullite, zirconia toughened alumina, spinel,aluminosilicates (e.g., mullite, cordierite), silicon carbide, siliconnitride, titanium carbide, titanium nitride, aluminum oxide, siliconoxide, zirconium oxide, stabilized zirconium oxide, aluminum carbide,aluminum nitride, zirconium carbide, zirconium nitride, aluminumoxynitride, silicon aluminum oxynitride, silicon dioxide, aluminumtitanate, tungsten carbide, tungsten nitride, steatite, and the like, orany combination thereof. Metallic particulates include, but are notlimited to, iron, nickel, chromium, silicon, aluminum, copper, cobalt,beryllium, tungsten, molybdenum, titanium, magnesium, silver, as well asalloys of metals, and the like, or any combination thereof. Metallicparticulates may also include the family of intermetallic materials,such as the iron aluminides, nickel aluminides, titanium aluminides, andthe like. Organic particulates include, but are not limited to,carbon-based structures such as nanotubes, nanorods, nanowires,nanospheres, microspheres, whiskers of oxide, fullerenes, carbon fibers,nomex fibers, and the like, or combinations thereof. Mineral-basedparticulates include, but are not limited to, such materials as kyanite,mica, quartz, sapphire, corundum, including the range of aluminosilicateminerals that display high hardness and strength. Single crystalmaterials can be used.

The alumina precursor can be or include aluminum hydroxide, bauxite,gibbsite, boehmite or diaspore or any combination thereof. The aluminaor alumina precursor can have any particle size distribution.

The proppants of the present invention can be made by taking a pluralityof synthetic templates or green body cores as described herein whichwould have a size, for instance, of from about 10 microns to about 30microns. This plurality of smaller green body cores can then be formedas part of a slurry and then a green body core comprising a plurality ofsmaller green body templates or cores can be formed having, forinstance, a diameter for this green body of from 20 microns to about 250microns. This green body can then be processed in the same manner asdescribed earlier to form a sintered ceramic proppant. The plurality ofsmaller templates or cores, during the sintering process, become onemass and ultimately form a sintered proppant that can have a hollow voidas described earlier. A small plurality of templates or cores can have ahollow central void or can be completely solid. FIG. 32 provides oneexample of a green body that is formed from a plurality of smallertemplate or core material. As can be seen in FIG. 32, 200 represents theoverall green body proppant that is formed from the plurality of smallertemplates or cores 202. 204 is part of the aqueous slurry that containsthe plurality of smaller templates/cores that ultimately will besintered.

In the present invention, the proppant can be made a number of ways,including, but not limited to, the following:

Option 1:

A solid green body core can first be made and while still a green body,a shell, or several shell layers can be formed on the green body core,and then the green body core/shell(s) can be sintered to form theceramic proppant. The green body core that is used in this option canthen remain solid or can form into a hollow void or core throughdiffusion during sintering, and the shell layer or layers can optionallycontain pore formers that create pores upon sintering and/or the shelllayer can contain microspheres. This two-step process can be used, forinstance, wherein a core can be formed, for instance, by spray-dryertechnique and then after the formation of the green body core, one ormore green shell layers can be formed, for instance, by fluid bedtechniques as described herein.

Option 2:

As another option, the green body core can be formed as above, but firstsintered to form a sintered core, which then can receive one or moreshell layers as described above in Option 1 and then sintered again.This core can also be a hollow core or a solid core.

Option 3:

A green body core and a green body shell can be formed at the same timeand the green body core can be hollow at the time of formation of thegreen body core/shell. For instance, this can be done by a co-axialmethod, such as co-axial extrusion or spray-drying or other techniquesthat can simultaneously or essentially simultaneously form a hollow coregreen body and one or more shell layers on top and then the overallproduct can be sintered. This would be a form of a one-step process.This one-step process can further have pore formers and/or microspherespresent in one or more shell layers as described, for instance, inOption 1.

Option 4:

A hollow core can be formed by using a fugitive spherical template, suchas a polymer template, such as a silicon-containing polymer. Thisfugitive spherical template can be a solid or a hollow fugitivespherical template and can be formed by co-axial nozzle techniques, suchas described herein. This fugitive spherical template can then have aceramic material applied on the surface so as to form a shell layers.One or more shell layers can be applied in this manner, such as by spraycoating ceramic mixture as described herein for the green body. Thenafterwards, the sintering can occur as described herein, wherein thefugitive template is burned out of a sintered ceramic proppant creatinga hollow central void. Interestingly, through sintering in an oxidizingatmosphere, the active polymeric template can be pyrolyzed and form SiO₂and/or other products which then, in turn, react with one or moreceramic components in the ceramic green shell material, such as alumina,to form a mullite inner layer or inner shell and an outer shell that isessentially the sintered ceramic shell. Put another way, as an option,the sintered proppant that is formed would essentially be a shell layerwith no ceramic core and would have at least two phases—one phase thatis a mullite-containing phase in the inner regions of the shell layerand a phase of ceramic that does not contain mullite.

The fugitive template as described above can be either solid or hollowand can be formed through an inkjet-like system with a piezoelectricdispensing mechanism using a solution of polymeric material, such aspolyethylene, poly(methyl) methacrylate, and the like. The pulsingpressure generated by the piezo device can break the continuous streamof the solution to droplets of essentially the same size. The surfacetension of the liquid then allows the droplets to become spherical andthe droplets can then be dried by appropriate techniques, such asfluidized bed spray drying techniques, drop tower drying techniques,infrared curing, UV curing, and the like. In the case of hollowmicrospheres, the nozzle can be co-axial and concentric with thesynchronized pulse gas (e.g., air) flow in the center and the liquidflow from the surrounding nozzle. FIG. 33 provides one example of themorphology of the fugitive polymeric templates formed and, in this case,polyethylene templates which, as can be seen, are extremely uniform withregard to size and shape. FIG. 34 shows a half of a synthetic ceramicproppant which was made using the polymeric templates of FIG. 33. Again,as can be seen, a very uniform central void can be achieved and a veryconsistent shell thickness around the entire sintered proppant can beachieved. Further, a proppant having very consistent shape and size thatmimics the fugitive templates can be achieved, as well, as shown in FIG.34. An illustrative example is provided as example 1.

For any one or more components that form the green body, for example,the particle size distribution, d_(as), can be from about 0.5 to about15, wherein, d_(as){(d_(a90)−d_(a10))/d_(a50)} wherein d_(a10) is aparticle size wherein 10% of the particles have a smaller particle size,d_(a50) is a median particle size wherein 50% of the particles have asmaller particle size, and d_(a90) is a particle size wherein 90% of theparticle volume has a smaller particle size. The d_(as) can be from 0.5to 15, 0.75 to 15, 1 to 15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10,0.5 to 5, and the like. The one or more components that make up thegreen body, such as alumina or alumina precursor, can have a medianparticle size, d_(a50), of from about 0.01 μm to about 100 μm, whereind_(a50) is a median particle size where 50% of the particles of thedistribution have a smaller particle size. The median particle size,d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1 to 90μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1 to 30μm, 1 to 20 μm, 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70 μm, andthe like, wherein d_(a50) is a median particle size where 50% of theparticles of the distribution have a smaller particle size.

Further, as an option, the particulate material or particles used toform the green body core and/or green body shell can be or have aunimodal particle distribution. In other words, the proppant cancomprise a plurality of micron particles that are sintered together,wherein the micron particles have a unimodal particle distribution. Themicron particles can have a d₅₀ of 0.5 micron to 3.5 microns.

The siliceous material that can be one or more of the components thatform the green body, can be any silicon containing material, such assilicate containing material, silicon containing minerals or ore,silicates, silicon oxides, and the like. The siliceous material can beor include one or more cenospheres, fly ash or any combination thereof.The siliceous material can be natural, synthetic, or a by-product. Thesiliceous material can be or include silicate materials, quartz,feldspar, zeolites, bauxite, calcined clays or any combination thereof.The siliceous material can have any particle size, such as a particlesize distribution, The d_(as) can be from 0.5 to 15, 0.75 to 15, 1 to15, 1 to 5, 1 to 6, 1 to 8, 5 to 15, 0.5 to 10, 0.5 to 5d_(ss), of fromabout 0.5 to about 15, wherein, d_(as)={(d_(s90)-d_(s10))/d_(s50)}wherein d_(s10) is a particle size wherein 10% of the particles have asmaller particle size, d_(s50) is a median particle size wherein 50% ofthe particles have a smaller particle size, and d_(s90) is a particlesize wherein 90% of the particle volume has a smaller particle size. Thed_(as) can be from 0.5 to 15, 0.75 to 15, 1 to 15, 1 to 5, 1 to 6, 1 to8, 5 to 15, 0.5 to 10, 0.5 to 5 and the like. The siliceous material canhave a median particle size, d_(a50), of from about 0.01 μm to about 100μm, wherein d_(a50) is a median particle size where 50% of the particlesof the distribution have a smaller particle size. The median particlesize, d_(a50), can be from about 1 μm to about 5 μm, from 1 to 5 μm, 1to 90 μm, 1 to 80 μm, 1 to 70 μm, 1 to 60 μm, 1 to 50 μm, 1 to 40 μm, 1to 30 μm, 1 to 20 μm to 1 to 10 μm, 10 to 90 μm, 20 to 80 μm, 30 to 70μm, and the like, wherein d_(a50) is a median particle size where 50% ofthe particles of the distribution have a smaller particle size.

As an option, the particle size distribution and/or the median particlesize of the alumina or precursor thereof and the siliceous materialand/or one or more other components that can be present, can be the sameor different, or can be within (±) 1%, 5%, 10%, 15%, 20%, 25% of eachother.

The green body material can include at least one binder. The binder canbe or include a wax, a starch, polyvinyl alcohol, a sodium silicatesolution, or a low molecular weight functionalized polymer (e.g., 1,000MW to 100,000 MW or 500 MW to 5,000 MW) or any combination thereof. Abinder may be used to facilitate the formation of the green bodymixture.

The green body material can further include at least one dispersant. Thedispersant can be or include at least one surfactant. A dispersant maybe used to facilitate a uniform mixture of alumina or alumina precursorand a siliceous material in the green body material. Specificdispersants can include, but are not limited to, DOLAPIX CE64 (Zschimmer& Schwarz, GmbH), DARVAN C(RT Vanderbilt Company, Industrial Minerals &Chemicals) and similar materials which may comprise from about 0% byweight to about 5% by weight of the green body material or any otheramount to assist in the dispersion of materials.

The green body material can further include at least one slurryingagent. The slurrying agent can be or include water, an organic solventor any combination thereof.

Besides the other ingredients mentioned above that can comprise theslurry, including the particulates (which includes the ceramic and/oroxide material), the binder, and dispersant, other optional componentscan be one or more of the following: flux agent (sodium silicate and/orsodium oxide), a defoaming agent (e.g., TU-44, or TU-45), and the like.An example of a binder is Optapix AC 112 or Optapix AC95 from Zschimmer& Schwartz. A suitable dispersant can be Dolapix CE-64 from Zschimmer &Schwartz. A rheological control agent (viscosifier) can also be presentas an option, which can be Bentone EW from Elementis. The rheologicalcontrol agent can be present in an amount, for instance, from 0.25 wt %to 1 wt % based on the overall weight of the slurry.

The slurry can have a variety of viscosities. Preferably, the viscosityof the slurry is such to obtain more uniform droplets and, therefore,obtain monodisperse microspheres. The viscosity is preferably in therange of from about 10² to about 10⁵ cP, such as 10¹ cP to 10³ cP. Otherexamples of viscosities can be from 10³ to 10⁴ cP.

With regard to the spray dryer, an example of a suitable spray dryer isa GEA Niro Mobile Minor or Anhydro spray dryer.

Upon exiting the spray dryer, the green body can optionally receive oneor more coatings that can form a shell using a fluid bed coater, forinstance, 100N manufactured by Applied Chemical Technologies, orVFC-1200 manufactured by Vector Corporation.

Upon exiting the spray dryer or fluid bed coater, the green body canthen be subjected to sintering.

The sintering can be performed under a pressure of from about 0.1×10⁵ Pato about 10×10⁵ Pa, such as from about 0.5×10⁵ Pa to about 7×10⁵ Pa, orfrom about 1×10⁵ Pa to about 5×10⁵ Pa.

The sintering can be performed at a temperature from about 500° C. toabout 2500° C. The sintering can be performed at an elevated pressure,for instance at a pressure from about 0.1 MPa to about 200 MPa for about1 hour to about 20 hours. The sintering preferably occurs at atemperature below 1400° C., such as from 1000° C. to about 1200° C., forabout 30 minutes to 4 hours, and more preferably from 2 to 4 hours. Thesintering temperatures referred to herein are the temperature of thematerial being sintered. Other sintering temperatures/times can be at atemperature from about 1100° C. to about 1300° C. for about 1 hour toabout 20 hours. Another example of the pressure during sintering is fromabout 0.1 MPa to about 200 MPa.

The sintering can be performed at any firing rate, such as a firing rateof from about 0.01° C./min to about 2000° C./min.

Sintering furnaces that can be used as a reactor in the present methodcan be any vessel that would permit the present method to be achieved.For instance, the reactor can be a fluidized bed furnace or fluidizedfurnace. The reactor can be a high temperature reactor, for instance,with process atmospheric control(s). Other types of furnaces can beused. The high temperature reactor can be a sealed chamber that permitscontrol of the process atmosphere (composition, pressure, and the like)and can be heated by any means, including, but not limited to, radiant,infra-red, microwave, induction, RF, laser, self propagating combustion,and the like. The fluidized bed furnace can use air or anoxygen-containing gas, or an inert gas as the fluidizing medium. Exampleof other furnaces (or reactors) include:

-   -   i. Rotary    -   ii. Static Bed (or other dynamic bed furnace)    -   iii. Muffle    -   iv. Drop Tower    -   v. Mechanical fluid bed where the air is recycled and/or    -   vi. Microwave,        -   These above furnaces generally use a sealed environment.    -   vii. Conventional fluidized bed furnace.

With regard to the formation of the green body template or core, asindicated, spray drying techniques can be used. As preferred options,the following is provided.

The slurry that is used to form the green body template or core can bean aqueous (or non-aqueous) suspension of oxide and/or non-oxide ceramicparticles. The particles can have a d₅₀ particle size ranging from 0.2micron to about 50 microns (e.g., 0.5 micron to 2.5 microns, 0.75 micronto 2 microns, 1 micron to 2 microns, 0.2 micron to 5 microns) or othersizes. The slurry can have a solids concentration of from about 30 wt %to about 80 wt %, such as from about 35 wt % to 75 wt %, 40 wt % to 70wt %, 45 wt % to 60 wt %, 50 wt % to 80 wt % based on the overall weightpercent of the slurry. The slurry can contain one or more binders, suchas one or more organic binders. The binders can be present in an amountfrom about 0.5 wt % to 5 wt % or other amounts, such as 1 wt % to 4 wt%, 2 wt % to 5 wt %, and the like. The weight percent is based on a drypowder basis (i.e., the dry components that form the slurry). As afurther option, the slurry can contain one or more dispersants and/orsurfactants, which can improve rheological properties (such asviscosity, stability, and the like) of the slurry. The dispersant can bepresent, for instance, in an amount of from 0.1 wt % to about 1.5 wt %,such as 0.1 wt % to 1.2 wt % and the like, based on a dry powder basis.

The spray dryer can have an inlet air temperature that ranges from 225°C. to 400° C. or other temperatures outside of this range. The spraydryer can have an outlet air temperature that ranges from 95° C. to 115°C. or other temperatures outside of this range. The spray dryer can havean atomizing air pressure that ranges from 0.2 bar to 2 bar or otherpressures above this range. The spray dryer can have a slurry flow ratethat ranges from 20 grams per minute to 9,000 grams per minute orhigher.

Described here is one option to preparing the slurry and synthetic greenbodies and proppants. The slurry can be made with desired ceramic matrixpowder having a desired particle size (e.g. average mean particle sized₅₀=1.50±0.15 μm or other sizes) optionally with at least one binderwith or without at least one defoamer.

The slurry can be sprayed through a nozzle under constant or pulsingdispensing pressure to form droplets that can immediately become spheresdue to the surface tension of the slurry. The nozzle may be of thesingle fluid hydraulic type, a two fluid nozzle in which compressed airis used to assist droplet formation and the two fluid nozzle may be ofthe internal mix or external mix variety. Other nozzle types may be usedincluding a design that incorporates a secondary “blowing” air stream toeffectively blow bubbles of slurry and thus form hollow spheres. Oneexample of a spray nozzle is set forth in FIG. 7. This spray nozzle 11has an air cap 1, a swirl plate 3, a slurry nozzle 5, a secondary nozzle7, and a nozzle body 9.

The spheres are then dried (preferably immediately) in a chamber filledwith blowing hot air, with the process operating in counter-currentmode. That is, the slurry droplet trajectory is in the oppositedirection to the hot air flow. The product fraction of interest iscollected at the bottom of the chamber by way of an airlock assembly.Particles that are below a critical size pass through the exhaust streamof the spray drier and are separated from the air stream by way ofvarious devices including, but not limited to, cyclones, bag dustcollector, electrostatic dust collectors, and the like.

The dried green products are then sintered at a temperature to densifyand strengthen the structure, as described earlier.

By changing the composition of the starting material in the slurry,porous spheres can be produced. For instance, the addition of fugitivephases can be used. The fugitive phase can be or include a combustibleinorganic or organic material. For instance, the combustible inorganicor organic material can be or include cellulose-based material,wood-based material, and/or carbonaceous material, polymeric material(or particles) or any combination thereof. The combustible inorganic ororganic material can be or include crushed tree nut shell material,carbon black, carbon fiber, charcoal, activated carbon, carbon toner,graphite, coal, paper, plant material, starch, starch granules, flour,or any combination thereof. International Patent Application WO2011/082102 provides techniques and materials that can be used here andthis application is incorporated in its entirety by reference herein.

By using a co-axial nozzle with different slurries, proppants withcore-shell structure can be produced simultaneously. For instance, thecenter orifice of the nozzle assembly may carry a cenosphere (or flyash)slurry and the outer slurry orifice of the nozzle assembly may carry thematrix ceramic slurry. By control of the two slurry flow rates andpressures and the atomizing air pressure, droplets of slurry consistingof a central region of cenosphere (or flyash) slurry encapsulated by theceramic matrix slurry may be formed, which then pass into the dryingchamber of the spray dryer and are formed into green sphericalparticles.

A multilayer core-shell structure can be produced by a co-axial nozzlespray process to obtain a functionally gradient structure for bettermechanical or chemical properties.

By using a co-axial nozzle, a green body with a hollow core in thecenter can be formed by a continuous or pulsing stream of air, and oneor more periphery hollow stream(s) to form a shell of simple matrix or acomplex shell with a functionally gradient matrix.

Regarding the sintering process, in more detail, the sintering can be afast heating process. A tunnel kiln can be used. Or, the particulateproppant can be sintered by a fast sintering technique with ramping rateup to 50° C./min or faster. The ramping rate can be 10 to 100° C./min oreven higher. In addition, the holding time can be reduced from severalhours to within one hour or even a few minutes only (e.g., 3 minutes to30 minutes). As indicated, the sintering can occur in fluidized bedconditions or in a rotary kiln. With the fast and homogeneous heating inthe sintering process, the mechanical properties of the product aresubstantially improved, because fast sintering can suppress grain growthand allow fine-grain microstructure. The fine-grain ceramics can bebeneficial to fracture toughness and strength.

The proppants of the present invention while preferably used to propopen subterranean formation fractions, can be used in othertechnologies, such as an additive for cement or an additive forpolymers, or other materials that harden, or would benefit. Theproppants of the present invention can also be used as encapsulateddelivery systems for drugs, chemicals, and the like.

The proppants of the present invention can be used to prop opensubterranean formation fractions. The proppant can be suspended in aliquid phase or other medium to facilitate transporting the proppantdown the well to a subterranean formation and placed such as to allowthe flow of hydrocarbons out of the formation. The medium chosen forpumping the proppant can be any desired medium capable of transportingthe proppant to its desired location including, but not limited to, agas and/or liquid, energized fluid, foam, like aqueous solutions, suchas water, brine solutions, and/or synthetic solutions. Any of theproppants of the present invention can have a crush strength sufficientfor serving as a proppant to prop open subterranean formation fractures.For instance, the crush strength can be 1,000 psi or greater, 3,000 psior greater, greater than 4,000 psi, greater than 9,000 psi, or greaterthan 12,000 psi. Suitable crush strength ranges can be from about 3,000psi to about 20,000 psi, or from about 5,000 psi to about 20,000 psi,and the like. In some applications, like coal bed methane recovery, acrush strength below 3,000 psi can be useful, such as 500 psi to 3,000psi, or 1,500 psi to 2,000 psi.

The proppant can be suspended in a suitable gas, foam, energized fluid,or liquid phase. The carrier material, such as a liquid phase isgenerally one that permits transport to a location for use, such as awell site or subterranean formation. For instance, the subterraneanformation can be one where proppants are used to improve or contributeto the flow of hydrocarbons, natural gas, or other raw materials out ofthe subterranean formation. The present invention also relates to a wellsite or subterranean formation containing one or more proppants of thepresent invention.

The proppants of the present invention also can present oil and gasproducers with one or more of the following benefits: improved flowrates, improved productive life of wells, improved ability to designhydraulic fractures, and/or reduced environmental impact. The proppantsof the present invention also can eliminate or materially reduce the useof permeability destroying polymer gels, and/or reduce pressure dropthrough the proppant pack, and/or the ability to reduce the amount ofwater trapped between proppants thereby increasing hydrocarbon “flowarea.”

The high density of conventional ceramic proppants and sands (roughly100 lb/cu·ft.) inhibit their transport inside fractures. High densitycauses proppants to “settle out” when pumped thereby minimizing theirefficacy. To maintain dense proppants in solution, expensive polymergels are typically mixed with the carrier solution (e.g. completionfluid). Once suspended in a gelled completion fluid, proppant transportis considerably enhanced. Polymer gels are extremely difficult tode-cross link, however. As a result, the gel becomes trapped downhole,coats the fracture, and thereby reduces reservoir permeability.Gel-related reservoir permeability “damage factors” can range from 40%to more than 80% depending on formation type. The lightweight highstrength buoyancy property that can be exhibited by the proppants of thepresent invention can eliminate or greatly reduce the need to employpermeability destroying polymer gels, as they naturally stay insuspension. The use of extreme pressure, polymer gels, and/or exoticcompletion fluids to place ceramic proppants into formations adverselyimpacts the mechanical strength of the reservoir and shortens itseconomic life. Proppants of the present invention can enable the use ofsimpler completion fluids and possibly less (or slower) destructivepumping. Thus, reservoirs packed with buoyant proppants preferablyexhibit improved mechanical strength/permeability and thus increasedeconomic life.

Enhanced proppant transport enabled by buoyancy also may enable theplacement of the present proppants in areas that were heretoforeimpossible, or at least very difficult to prop. As a result, themechanical strength of the formation can be improved, and can reducedecline rates over time. This benefit could be of significantimportance, especially within hydraulic fractures (“water fracs”) wherethe ability to place proppants can be extremely limited. If neutrallybuoyant proppants are employed, for example, water (fresh to heavybrines) may be used in place of more exotic completion fluids. The useof simpler completion fluids can reduce or eliminate the need to employde-crossing linking agents. Further, increased use of environmentallyfriendly proppants may reduce the need to employ other environmentallydamaging completion techniques such as flashing formations withhydrochloric acid. In addition to fresh water, salt water and brines, orsynthetic fluids are sometimes used in placing proppants to the desiredlocations. These are of particular importance for deep wells.

While the term proppant has been used to identify the preferred use ofthe materials of the present invention, it is to be understood that thematerials of the present invention can be used in other applications.The proppant of the present invention also can be used to form otherproducts, such as, for example, matrix materials, concrete formulations,composite reinforcement phase, thermal insulating material, electricalinsulating material, abrasive material, catalyst substrate and/orsupport, chromatography column materials (e.g., column packings), refluxtower materials (e.g., reflux tower packings, for instance, indistillation columns), and the like. The proppants may be used inmedical applications, filtration, polymeric applications, catalysts,rubber applications, filler applications, drug delivery, pharmaceuticalapplications, and the like.

The present invention has many advantages, including achieving amonodisperse distribution and/or providing enhanced conductivity and/orpermeability, mechanical properties enhancement through microstructuralcontrol, and/or case strengthening by core material diffusion, and/orcontrol over defect distribution either by elimination or filling ofdefects by core material during diffusion or both, and the like.

The present invention will be further clarified by the followingexamples, which are intended to be exemplary of the present invention.

EXAMPLES Example 1

In order to evaluate the effect of the desired hollow synthetic templateon the mechanical strength of the proppant made with it, a comparativestudy was carried out with a sacrificial polymeric template andcommercially available cenosphere template from Cospheric, LLC, SantaBarbara, Calif. The synthetic proppant (to form a proppant or anon-sacrificial template) was made by spray coating a slurry (asdescribed in Table 8) on a substantially monodisperse highly sphericalpolyethylene microsphere having an average particle size of 215 micronsthat was commercially available, followed by burnout of the polyethylenecore under a slow heating process and then sintering. The resultantsynthetic ceno microspheres were highly spherical, narrow in particlesize distribution, and uniform in shell thickness. Proppant sample wasmade by spray coating of a ceramic slurry on the hollow synthetictemplate, whereas a control was made by same spray coating of the slurryon the cenosphere template. The samples were sintered for 2 hours at1250° C. Single sphere crush test was carried out to evaluate themechanical strength of the samples with 30 sintered proppant beads foreach sample. The results are shown in Table 1. It is seen that with thesame average sphere size (358 μm), the sample of synthetic template was58% stronger than the control, even with lower Sg for the synthetictemplate sample (2.49 vs. 2.56). Since everything else except thestructural defects for the sample and the control are the same, thesignificant improvement in the crush strength of the proppant with thesynthetic template is attributed to the substantial elimination ofstructural defects in the synthetic template.

TABLE 1 Single sphere crush strength of the proppant coated on synthetictemplate (ST) vs. control coated on regular cenosphere template, bothsintered for 2 h at 1250° C. System Sg Size (μm) Crush strength (N)Improvement Control 2.56 358 ± 11 18.6 ± 2.9 Control ST Proppant 2.49358 ± 9  29.4 ± 4.1 58%

Example 2

A slurry of ceramic powder with the following chemical composition(Table 2) and mixing proportions (Table 3) was milled to an averageparticle size d₅₀=1.5 μm. The slurry was then used to make microspheresby spray drying process. The typical morphology of the sample is shownin FIGS. 1-3. FIG. 4 shows the influence of inlet temperature on theparticle size distribution of the sintered product. The average particlesizes are listed in Table 4 and 5 with binders AC-112 and AC-95,respectively.

TABLE 2 Chemical composition of ceramic powder Composition SiO₂ Al₂O₃Fe₂O₃ MgO CaO Na₂O K₂O TiO₂ P₂O₅ Others Wt. % 61.35 24.56 5.08 1.53 1.581.01 2.51 0.95 0.19 1.24

TABLE 3 Mixing proportions of spray slurry Composition Ceramic powderDispersant Water Binder Wt. % 50 0.5 46.5 3.0

These results show that mean particle size of the synthetic templates isdependent upon both the inlet temperature and the outlet temperature.The outlet temperature, for a given inlet temperature, is controlled bythe slurry flow rate, and assuming that the nozzle air pressure remainsconstant, the droplet size will change dependent upon the slurry flowrate, i.e. higher inlet air temperature dictates the use of a higherslurry flow rate to maintain outlet air temperature, by way ofevaporative cooling of the process air stream.

TABLE 4 Average sintered particle size (d₅₀) in microns influenced byoperational parameters (T_(in) = inlet temperature; T_(out) = outlettemperature) with 3% AC-112 binder T_(out)↓ T_(in) = 275° C. T_(in) =300° C. T_(in) = 325° C. 100° C. 37.1 36.4 41.3 105° C. 30.1 31.9 30.9110° C. 29.2 27.5 28.2

TABLE 5 Average sintered particle size (d₅₀) in microns influenced byoperational parameters (T_(in) = inlet temperature; T_(out) = outlettemperature) with 5% AC-95 binder T_(out)↓ T_(in) = 275° C. T_(in) =300° C. T_(in) = 325° C. 100° C. 33.9 40.0 48.4 105° C. 33.5 38.7 47.8110° C. 33.7 37.5 47.2

As can be seen in Table 4, by adjusting the inlet temperature, theaveraged sintered particle size of the proppant or proppant template canbe controlled. For instance, as the inlet temperature is increased, theaveraged sintered particle size can be increased to a certain extent.Similarly, the outlet temperature, as it is increased, can decrease theaveraged sintered particle size. Similar results are shown in Table 5,where more binder was present in the green body and a significant changein the d₅₀ size of the sintered particle occurred due to changing theinlet temperature. As stated, Tables 4 and 5 show the averaged sinteredparticle size, but the reference to inlet temperature and outlettemperature is with respect to the spray dryer and the processing of thegreen body. In these examples, the sintering of the green body occurredat 1,000° C. for 30 minutes.

Example 3

In these examples, various slurries were prepared for spray drying inorder to make ceramic green bodies that ultimately formed the core. InTable 6 below, the slurry was prepared by milling the additives thatcomprised the slurry to achieve a d₅₀ of 1.5 microns. Then, the milledadditives were added to water to form a slurry. The slurry in Table 6had the following ingredients:

Crushed TG-425 cenospheres

Dispersant (Dolapix CE-64)

Binder (Optapix AC95 or Optapix AC112)

Water.

Table 6 sets forth the binder content, viscosity, density, solid weightpercent, and surface tension, as well as the Z number.

Further, Table 7 below provides examples of slurry which had poorsprayability based on observed results. The slurry used was alsoprepared by milling the ingredients to have a d₅₀ of about 1.5 micronsand then forming a slurry as above. The slurry had the followingingredients:

Flyash

Dispersant (Dolapox CE-64)

Binder (Optapix AC95 or Optapix AC112)

Water

TABLE 6 Examples of slurries with good workability for spray dryingSlurry d₅₀, μm Binder Solid % η σ ρ Z 1 2.13 AC-95 52.9 1.71 0.060 145812.88 2 2.13 AC-95 54.7 2.25 0.063 1460 16.64 3 2.13 AC-112 53.4 2.070.072 1460 14.25 4 2.13 AC-112 54.6 2.25 0.093 1459 13.67 Notes: ηrepresents viscosity in Pa · s, measured at 20 RPM; σ, surface tensionin N/m; ρ, density of the slurry, kg/m³; Z, Ohnesorge number,dimensionless.

TABLE 7 Examples of slurries with poor sprayability (must be modified tobe spray dried) Slurry d₅₀, μm Binder (%) Solid % η σ ρ Z 1 1.50 AC-95(5) 54.7 0.056 0.099 1510 0.33 2 1.50 AC-95(7) 54.8 0.053 0.104 14900.30 3 1.50 AC-112(5) 55.9 0.030 0.099 1490 0.17 4 1.50 AC-112(7) 55.50.039 0.133 1490 0.20 Notes: η represents viscosity in Pa · s, measuredat 20 RPM; σ, surface tension in N/m; ρ, density of the slurry, kg/m³;Z, Ohnesorge number, dimensionless.

Thus, as shown in this example, various parameters can affect desirableproperties, such as the ability to obtain monodispersed and highspherical proppants on a uniform basis.

Example 4

As set forth in Table 8 below, six proppants were made from a slurryformulation specified in the table In each case, the solids content ofthe slurry formulation was 36 wt %. In addition, the coating slurrycontained 22% perlite, 63% alumina, 15% nepheline syenite, and 5% ballclay, each based on wt % based on the overall weight of the slurry.Table 8 also sets forth the d₅₀, particle size for the slurryformulation. In addition, Table 9 sets forth the sintered body d₅₀ size,the green and sintered body sphericity based on Krumbein, and theresulting sintered size provided in both μm and mesh. Also, Table 9 setsforth the amount of crush fines on average generated from a crushstrength test of 20,000 psi and a crush strength test based on 25,000psi following API RP-60 standard. Finally, the resulting d₅₀ size of thesintered proppant, as well as the sphericity of the sintered proppantand the standard deviation (Sigma), is provided. As can be seen, thegreen body sphericity, as well as the sintered proppant sphericity, wereimmensely high and thus close to one for sphericity. Further, thefinished Sigma (the Sigma of the sintered product) was an extremelytight particle distribution, thus showing the highly consistentproppant, that can be made in the methods of the present application.Further to this, Table 10 presents the data for single sphere strengthmeasurement data for proppants comparing the control sample which isbased on the coating of cenospheres with the comparative proppant sampleutilizing synthetic templates, highlighting the significant strengthincreases in the proppant that are realized from the use of synthetictemplates. Table 11 presents the measured data for the crush testresults for as assemblage of proppant particles tested according to APIRP-60. Of note is the significant increase in crush strength at bothtest loadings of 15 ksi and 20 ksi.

TABLE 8 Components of the slurry for spray coating on synthetic template(wt. %) Perlite Alumina Nepheline Syenite d₅₀, μm Solid content 22% 63%15% 1.5 36%

TABLE 9 Properties of the proppants prepared with the spray driedsynthetic template Sphericity Crush fines, % Serial No. Code SG Meshd₅₀, μm Green Sintered 20 ksi 25 ksi 1 VBP394A 2.80 40/50 325 ± 23 0.9710.967 4.31 7.37 2 VBP397A 2.89 40/50 321 ± 24 0.971 0.968 3.54 6.26 3VBP401A 2.89 40/50 320 ± 28 0.968 0.969 4.35 7.78 4 VBP394B 2.84 30/40475 ± 28 0.974 0.970 5.63 8.22 5 VBP397B 2.97 30/40 482 ± 30 0.973 0.9705.43 9.39 6 VBP401B 2.95 30/40 475 ± 35 0.976 0.970 5.82 10.13

TABLE 10 Single sphere strength of the proppant based on synthetictemplate Vs cenosphere (control) Sample # Type No. tested Size, μm Crushstrength, N Δ % 4070C Control 30 355.3 ± 10.7 26.4 ± 5.2  — 4070S Sample30 350.0 ± 11.7 30.9 ± 5.2  17.0 3050C Control 30 495.0 ± 10.1 60.1 ±10.5 — 3050S Sample 30 495.0 ± 11.1 71.1 ± 12.1 18.3

TABLE 11 Crush test results of proppant based on synthetic template vs.cenosphere (control) Code Note Sg 15 ksi fines % 18 ksi fines % 4070CCeno 2.83 3.95 ± 0.35 6.07 ± 0.87 4070S Sample 2.94 3.10 ± 0.14 3.85 ±0.07 Difference Δ % +3.52 −21.52 −36.57 3050C Control 2.90 5.40 ± 0.14 7.5 ± 0.42 3050S Sample 2.99 4.75 ± 0.07 5.95 ± 0.07 Difference Δ %+3.1 −12.03 −20.67

Example 4

An aqueous slurry of flyash (Bowen UFF) with a 5 wt % super addition ofball clay was milled to a mean particle size (d₅₀) of approximately 1.5microns and a solids loading of 52 wt %. No organic binder was added tothis slurry prior to spray drying. Spray drying was carried out in aGEA/Niro Mobile Minor spray dryer with the following parameters: Inlettemperature 280° C., Outlet Temperature 116° C., atomizing air pressure0.4 bar, slurry flow rate approximately 0.25 kg/min, using a two-fluidair atomizing nozzle fitted to the spray dryer in a counter current (orfountain) orientation. Examination of the spheres generated during thespray drying process revealed the formation of phases as layers on thesphere. The segregation of the lighter residual carbon phase towards thesurface of the sphere occurred during the drying operation withconcentration of the segregated carbon to a localized area on thesurface of the sphere.

The present invention includes the followingaspects/embodiments/features in any order and/or in any combination:

1. The present invention relates to a plurality of sintered ceramicproppants having a mean particle size, wherein the sintered ceramicproppants are monodispersed with a distribution that is a 3-sigmadistribution or lower with a width of the total distribution being 5% orless of the mean particle size.

2. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said distribution is a2-sigma distribution or lower.

3. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein the distribution is a1-sigma distribution.

4. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise aluminum oxide, silicon dioxide, and one or moremixed metal aluminum oxides.

5. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants have a specific gravity of from 0.6 to 4.

6. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said proppants have a crushstrength of from 5,000 psi to 30,000 psi.

7. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants have a Krumbein sphericity of at least 0.9.

8. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants have a particle size of from about 100 microns to 3,000microns.

9. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise a core and at least one shell around said core.

10. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise a core and a shell, wherein a central void is presentwithin said core.

11. A method of making a sintered ceramic proppant comprising forming aspherical green body core comprising one or more ceramic particulatematerials;

-   -   forming, at the same time or afterwards, a green body shell        around said green body core, wherein said green body shell        comprises at least one ceramic particulate material which        results in a green core/shell body;    -   sintering said green core/shell body, and, during sintering,        diffusing at least a portion of said green body core into said        green body shell to form a sintered ceramic proppant having a        central void and shell.

12. The method of any preceding or following embodiment/feature/aspect,wherein said central void comprises at least 5% by volume of the overallvolume of the sintered ceramic proppant.

13. The method of any preceding or following embodiment/feature/aspect,wherein said diffusing results in at least 50% by weight of said greenbody core diffusing into said shell.

14. The method of any preceding or following embodiment/feature/aspect,wherein said diffusing results in at least 80% by weight of said greenbody core diffusing into said shell.

15. The method of any preceding or following embodiment/feature/aspect,wherein said diffusing results in at least 99% by weight of said greenbody core diffusing into said shell.

16. The method of any preceding or following embodiment/feature/aspect,wherein the green body shell has a softening temperature that is higherthan the softening temperature of the green body core.

17. The method of any preceding or following embodiment/feature/aspect,wherein said green body shell has a softening temperature of at least100° C. higher than the softening temperature of the green body core.

18. The method of any preceding or following embodiment/feature/aspect,wherein the softening temperature of the green body shell is from about300° C. to about 400° C. higher than the softening temperature of thegreen body core.

19. The method of any preceding or following embodiment/feature/aspect,wherein the green body shell has a porosity of at least 10% by volumebased on the volume of the green body shell.

20. The method of any preceding or following embodiment/feature/aspect,wherein the green body shell has a porosity of at least 30% by volumebased on the volume of the green body shell.

21. The method of any preceding or following embodiment/feature/aspect,wherein said sintered ceramic proppant has at least 10% porosity in thesintered shell.

22. A plurality of sintered ceramic proppants having a Krumbeinsphericity of at least 0.92.

23. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said Krumbein sphericity is0.95 to 0.99.

24. The method of any preceding or following embodiment/feature/aspect,wherein the slurry has an Ohnesorge Number (Z) of from 1 to 10.

25. The method of any preceding or following embodiment/feature/aspect,wherein the slurry has an Ohnesorge Number (Z) of from 2 to 10.

26. The method of any preceding or following embodiment/feature/aspect,wherein the slurry has an Ohnesorge Number (Z) of from 4 to 6.

27. A proppant comprising a plurality of micron particles that aresintered together, wherein said micron particles have a unimodalparticle distribution, wherein said micron particles have a d₅₀ of 0.5micron to 3.5 microns.

28. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a plurality ofpores having a pore volume wherein a majority of the pore volume resultsfrom interstitial gaps formed between the micron particles.

29. The proppant of any preceding or followingembodiment/feature/aspect, wherein the proppant is spherical and have aKrumbein sphericity of at least about 0.9 and/or a roundness of at leastabout 0.9.

30. The proppant of any preceding or followingembodiment/feature/aspect, wherein the pore volume is from about 1% to30% based upon the total volume of said proppant.

31. The proppant of any preceding or followingembodiment/feature/aspect, wherein the pore volume is from about 5% to20% based upon the total volume of said proppant.

32. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a specific gravityof from 0.8 to 4.

33. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a specific gravityof from about 1 to 3.5.

34. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a d₁₀ that iswithin 100% of the d₅₀.

35. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a d₁₀ that iswithin 50% of the d₅₀.

36. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a d₉₀ that iswithin 100% of the d₅₀.

37. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a d₉₀ that iswithin 50% of the d₅₀.

38. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a d₁₀ that iswithin 100% of the d₅₀ and has a d₉₀ that is within 100% of the d₅₀.

39. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a core and at leastone shell on said core.

40. The proppant of any preceding or followingembodiment/feature/aspect, wherein said core comprises said plurality ofmicron particles that are sintered together.

41. The proppant of any preceding or followingembodiment/feature/aspect, wherein said shell comprises a plurality ofmicron particles that are sintered together.

42. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant is in the absence of abinder.

43. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant is in the absence of apolymer.

44. The proppant of any preceding or followingembodiment/feature/aspect, wherein the core comprises a plurality ofmicron particles that are sintered together, wherein said micronparticles have a bimodal particle distribution with a modal A particledistribution and a modal B particle distribution.

45. The proppant of any preceding or followingembodiment/feature/aspect, wherein said micron particles of each modalhave a d₅₀ of 0.5 micron to 3.5 microns, and modal A has a d₅₀ that isat least 10% different from the d₅₀ of modal B.

46. The proppant of any preceding or followingembodiment/feature/aspect, wherein said micron particles of each modalhave a d₅₀ of 0.5 micron to 3.5 microns, and modal A has a d₅₀ that isat least 20% different from the d₅₀ of modal B.

47. The proppant of any preceding or followingembodiment/feature/aspect, wherein said micron particles of each modalhave a d₅₀ of 0.5 micron to 3.5 microns, and modal A has a d₅₀ that isfrom 10% to 100% different from the d₅₀ of modal B.

48. A proppant comprising a core and a shell, wherein said core is aceramic or oxide core, and said shell comprises at least one ceramicmaterial, and said proppant has a core strength to shell strength ratioof from 0.8 to 1.

49. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has an overall proppantstrength to core strength ratio of from 2 to 3.

50. The proppant of any preceding or followingembodiment/feature/aspect, wherein said proppant has a specific gravityof 2.6 to 4.5.

51. The proppant of any preceding or followingembodiment/feature/aspect, wherein core is a synthetic core.

52. The method of any preceding or following embodiment/feature/aspect,wherein said green core is solid prior to said sintering.

53. The method of any preceding or following embodiment/feature/aspect,wherein said central void has a shape and size of said green core or aportion thereof.

54. The method of any preceding or following embodiment/feature/aspect,wherein whiskers or fibers are formed in-situ in said shell during saidsintering and as a result of said diffusing.

55. The method of any preceding or following embodiment/feature/aspect,wherein said diffusing of the green body core or portion thereof intothe shell results in a gradient of wherein a higher concentration of thecore is present closer to the core than to an exterior outer surface ofthe proppant.

56. The method of any preceding or following embodiment/feature/aspect,wherein said spherical green body, green body shell, or both furthercomprise at least one nucleating agent.

57. The method of any preceding or following embodiment/feature/aspect,wherein said ceramic particulate materials comprise cordierite, mullite,bauxite, silica, spodumene, silicon oxide, aluminum oxide, sodium oxide,potassium oxide, calcium oxide, zirconium oxide, lithium oxide, ironoxide, spinel, steatite, a silicate, a substituted alumino silicateclay, an inorganic nitride, an inorganic carbide, a non-oxide ceramic orany combination thereof.

58. The method of any preceding or following embodiment/feature/aspect,wherein said ceramic particulate materials comprise one or moresedimentary materials or synthetically produced materials or both.

59. The method of any preceding or following embodiment/feature/aspect,wherein said spherical green body core and said green body shell are inthe absence of igneous or metamorphic materials.

60. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants have less than 1% by weight of proppant of igneous ormetamorphic materials.

61. The method of any preceding or following embodiment/feature/aspect,wherein the green body or a portion thereof has a density, as measuredby a gas pycnometer, such that the average density (g/cm³) does notalter by more than 1% between the density of the whole green bodycompared to the density of the crushed green body.

62. The method of any preceding or following embodiment/feature/aspect,wherein the average density changes 0.005% or less.

63. The method of any preceding or following embodiment/feature/aspect,wherein one or more mobile phases are formed in droplets of the slurrythat forms the green body and one phase migrates to the surface of thedroplet, which causes a multi-phase droplet to form.

64. The method of any preceding or following embodiment/feature/aspect,wherein said multi-phase droplet forms a non-uniform green body ofphases.

65. The method of any preceding or following embodiment/feature/aspect,wherein said non-uniform green body of phases diffuses at differentrates into said shell with respect to the phases.

66. The method of any preceding or following embodiment/feature/aspect,wherein said green body core comprises at least 50% by weight, based onthe weight of the green body core of glassy material, and said greenbody shell comprises at least 50% crystalline material.

67. The method of any preceding or following embodiment/feature/aspect,wherein said green body core comprises at least 75% by weight, based onthe weight of the green body core of glassy material, and said greenbody shell comprises at least 75% crystalline material.

68. The method of any preceding or following embodiment/feature/aspect,wherein said green body core comprises at least 95% by weight, based onthe weight of the green body core of glassy material, and said greenbody shell comprises at least 95% crystalline material.

69. The method of any preceding or following embodiment/feature/aspect,wherein the particles used to form the green body core are at least 10%smaller in average mean size (d₅₀ size) compared to the mean particlesize (d₅₀ size) of the particles that form the green body shell.

70. The method of any preceding or following embodiment/feature/aspect,wherein the particles used to form the green body core are at least 50%smaller in average mean size (d₅₀) size) compared to the mean particlesize (d₅₀ size) of the particles that form the green body shell.

71. The method of any preceding or following embodiment/feature/aspect,wherein the particles used to form the green body core are at least 100%smaller in average mean size (d₅₀ size) compared to the mean particlesize (d₅₀ size) of the particles that form the green body shell.

72. The method of any preceding or following embodiment/feature/aspect,wherein the ceramic particulate materials that form the green body or apart thereof has the following standard deviation range based on theindicated mean particle size range:

-   -   100-299 μm, σ=0.83-2.5    -   300-499 μm, σ=2.5-4.16    -   500-799 μm, σ=4.16-6.66    -   800-999 μm, σ=6.66-8.33    -   1000-1499 μm, σ=8.33-12.5    -   1500-2000 μm, σ=12.5-16.66.

73. The method of any preceding or following embodiment/feature/aspect,wherein the ceramic particulate materials that form the green body or apart thereof has a monodisperse particle distribution such that

$d_{s} = \frac{\left( {d_{90} - d_{10}} \right)}{d_{50}}$

where d₉₀, d₅₀ and d₁₀ are the 90^(th), 50^(th), and 10^(th) percentilesof the particle size distribution respectively, wherein 0.00<d_(s)≦0.05.

74. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise at least one ceramic, wherein said ceramic comprisescordierite, mullite, bauxite, silica, spodumene, clay, silicon oxide,aluminum oxide, sodium oxide, potassium oxide, calcium oxide, zirconiumoxide, lithium oxide, iron oxide, spinel, steatite, a silicate, asubstituted alumino silicate clay, an inorganic nitride, an inorganiccarbide or a non-oxide ceramic or any mixtures thereof.

75. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise at least one ceramic, wherein said ceramic comprisesa glass-ceramic.

76. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said sintered ceramicproppants comprise at least one ceramic, wherein said ceramic comprisesaluminum oxide, silicon oxide, titanium oxide, iron oxide, magnesiumoxide, calcium oxide, potassium oxide and/or sodium oxide, or anycombination thereof.

77. A plurality of sintered ceramic proppants having a mean particlesize, wherein the sintered ceramic proppants are monodispersed with astandard deviation of 3 or less.

78. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said standard deviation is2.75 or less.

79. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said standard deviation is2 or less.

80. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said standard deviation is1 or less.

81. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said standard deviation is0.5 or less.

82. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said standard deviation isfrom 0.5 to 3.

83. A plurality of ceramic proppants having a mean particle size,wherein the ceramic proppants are monodispersed and have a coefficientof variance (CV) of 8% or less.

84. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said coefficient of variance is fromabout 5% to 8%.

85. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppants are sintered.

86. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppants are greenbodies.

87. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppants are greenbodies having a core and shell.

88. A ceramic proppant that comprises at least one ceramic, wherein saidproppant has a change in sphericity of 5% or less.

89. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said change of sphericity is 3% orless.

90. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said change of sphericity is fromabout 0.5% to 5%.

91. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant is sintered.

92. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant is a greenbody.

93. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant is a green bodyhaving a core and shell.

94. A ceramic proppant comprising at least one ceramic and having astrength/porosity relationship at a load of 20,000 psi of from 0.4 to0.9.

95. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said strength/porosity relationshipat a load of 20,000 psi is from 0.46 to 0.88.

96. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said strength/porosity relationshipat a load of 20,000 psi is from 0.5 to 0.8.

97. A ceramic proppant comprising at least one ceramic and having ameasured specific gravity that is within 10% of a specific gravitycalculated from a measured bulk density of the ceramic proppant.

98. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said measured specific gravity iswithin 5% of the specific gravity calculated from the measured bulkdensity.

99. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said measured specific gravity iswithin 1% of the specific gravity calculated from the measured bulkdensity.

100. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said measured specific gravity iswithin 0.1% of the specific gravity calculated from the measured bulkdensity.

101. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant has a maximumload of at least 18 N.

102. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant has a maximumload of from 20 N to 100 N.

103. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said ceramic proppant has a maximumload of from 40 N to 80 N.

104. A plurality of sintered ceramic proppants comprising at least oneceramic, wherein said plurality of proppants have an average crushstrength in psi as determined per single proppant and a coefficient ofvariance of the proppants for individual crush strength is 20% or less.

105. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said coefficient ofvariance is from 5% to 20%.

106. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said coefficient ofvariance is from 5% to 15%.

107. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said coefficient ofvariance is from 10% to 20%.

108. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said plurality is at leastone kilogram of proppant.

109. A plurality of sintered ceramic proppants comprising at least oneceramic, wherein said plurality of proppants have a coefficient ofvariance for size (size CV) of 10% or less, and the same plurality ofproppants have a coefficient of variance for the shape (shape CV) of 5%or less.

110. The plurality of sintered proppants of any preceding or followingembodiment/feature/aspect, wherein the sintered proppants have a sphereshape.

111. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said plurality of proppantshave said coefficient of variance for size (size CV) of 1% to 10%, andthe same plurality of proppants have said coefficient of variance forthe shape (shape CV) of 0.5 to 5%.

112. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said plurality of proppantshave said coefficient of variance for size (size CV) of 1% to 6%, andthe same plurality of proppants have said coefficient of variance forthe shape (shape CV) of 0.5 to 3%.

113. The plurality of sintered ceramic proppants of any preceding orfollowing embodiment/feature/aspect, wherein said plurality of proppantshave said coefficient of variance for size (size CV) of 3% to 8%, andthe same plurality of proppants have said coefficient of variance forthe shape (shape CV) of 0.5 to 3%.

114. A sintered ceramic proppant comprising at least one ceramic, and aceramic core that is synthetic and at least one ceramic shell, whereinsaid ceramic core, at a 20,000 psi crush test under API 60, has a 20,000psi crush fines that average 5.5% or less.

115. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said 20,000 psi crush fines average3% or less.

116. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said 20,000 psi crush fines averagefrom 0.5% to 5%.

117. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein the ceramic core has a sintered d₅₀size of 500 microns or less.

118. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein the ceramic core has a sintered d₅₀size of from 100 microns to 500 microns.

119. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein the ceramic core has a sintered d₅₀size of from 300 microns to 475 microns.

120. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein the ceramic core has a sintered d₅₀size of 500 microns or less and a specific gravity of 3 sg or lower.

121. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein the ceramic core has a sintered d₅₀size of 500 microns or less and a specific gravity of from 2 sg to 2.9sg.

122. A plurality of ceramic proppants having a crush resistance numberbased on the overall crush fine ratio, where

crush resistance Number (CR)={[D×Sd ₅₀ ]/[CF×P]}×10⁶

-   -   wherein CF represents the amount (by weight % in fraction) of        the crushed fines from a 20,000 psi crush test and is an average        and based on API RP-60, and weight % is based on the total        amount of particles being subjected to the crush test, D        represents the density of the proppants in g/cm³, Sd₅₀        represents sintered d₅₀ size of the proppants in microns, and P        is crush fine measurement pressure in g/cm², and wherein said        crush resistance number is from 0.5 to 3.

123. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said crush resistance number is from0.75 to 2.5.

124. The plurality of ceramic proppants of any preceding or followingembodiment/feature/aspect, wherein said crush resistance number is from1 to 2.

125. A ceramic proppant comprising a ceramic synthetic core or template,wherein said ceramic proppant has a strength to porosity ratio,determined by measuring crush strength (psi) of the ceramic proppant anddividing by amount of porosity (% volume) (including any central void)that is present in the ceramic proppant, and said strength to porosityratio is from 5×10⁴ to 50×10⁴.

126. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said strength to porosity ratio isfrom 5×10⁴ to 30×10⁴.

127. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said strength to porosity ratio isfrom 15×10⁴ to 30×10⁴.

128. The ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein said strength to porosity ratio isfrom 5×10⁴ to 10×10⁴.

129. A sintered ceramic proppant that is spherical and having a centralvoid, and having regions A to B, B to C and C to D, wherein region A toB is closest to the central void and region C to D is furthest away fromsaid central void, and region B to C is radially located between regionA to B and C to D and said sintered ceramic proppant having porositythat is highest in the central location of the shell with regard toradius of sintered ceramic proppant with region A to B having from 0% to5% (by volume of that region) of porosity, region B to C having porosityof from 5% to 30% by volume of that region, and region C to D havingporosity that is ±10% of region A to B.

130. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein region B to C has more porosity byvolume than region A to B and/or region C to D.

131. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein region B to C has at least 10% moreporosity than other said regions.

132. The sintered ceramic proppant of any preceding or followingembodiment/feature/aspect, wherein region A to B comprises from 10% to40% by volume of the overall non-void region of the proppant, region Bto C comprises from 20% to 50% by volume of the overall non-void regionsof the proppant and region C to D comprises from 10% to 40% by volume ofthe overall non-void regions of the proppant.

133. The method of any preceding or following embodiment/feature/aspect,wherein said slurry has a viscosity of from about 10² to about 10⁵ cP.

134. The method of any preceding or following embodiment/feature/aspect,wherein said sintering is performed under pressure at from about 0.1×10⁵to about 10×10⁵ Pa.

135. The method of any preceding or following embodiment/feature/aspect,wherein said ceramic particulate material have a d₅₀ particle size offrom 0.2 micron to about 50 microns.

136. The method of any preceding or following embodiment/feature/aspect,wherein said ceramic particulate material have a d₅₀ particle size offrom 0.5 micron to about 5 microns.

137. The method of any preceding or following embodiment/feature/aspect,wherein said ceramic particulate material have a d₅₀ particle size offrom 0.5 micron to about 2.5 microns.

138. A method of making a ceramic proppant comprising:

-   -   a. forming a green body core from a first plurality of particles        that comprise at least one type of first ceramic material;    -   b. forming at least one green shell layer around said green body        core to obtain a green body, wherein said green shell layer is        formed from a second plurality of particles that comprise at        least one type of second ceramic material, wherein said first        ceramic material and said second ceramic material is the same or        different; and    -   c. sintering said green body to form a sintered body.

139. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the green body core comprises spray drying aslurry containing said first plurality of particles into the shape ofsaid green body core.

140. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the at least one green shell layer comprisesutilizing a fluid bed to apply said second plurality of particles toprovide said green shell layer.

141. The method of any preceding or following embodiment/feature/aspect,wherein said second plurality of particles further comprises at leastone pore former or microsphere or both.

142. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a solid core with no central void.

143. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a hollow core having a central void.

144. A method of making a ceramic proppant comprising:

-   -   a. forming a green body core from a first plurality of particles        that comprise at least one type of first ceramic material;    -   b. sintering said green body core to form a sintered core;    -   c. forming at least one green shell layer around said sintered        core to obtain at least one green shell layer, wherein said        green shell layer is formed from a second plurality of particles        that comprise at least one type of second ceramic material,        wherein said first ceramic material and said second ceramic        material is the same or different;    -   d. sintering said at least one green shell layer to form a        sintered body having a core/shell.

145. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the green body core comprises spray drying aslurry containing said first plurality of particles into the shape ofsaid green body core.

146. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the at least one green shell layer comprisesutilizing a fluid bed to apply said second plurality of particles toprovide said at least one green shell layer.

147. The method of any preceding or following embodiment/feature/aspect,wherein said second plurality of particles further comprises at leastone pore former or microsphere or both.

148. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a solid core with no central void.

149. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a hollow core having a central void.

150. A method of a making ceramic proppant comprising:

-   -   a. forming at the same time or about the same time, a green body        core from a first plurality of particles that comprise at least        one type of first ceramic material and forming at least one        green shell layer around said green body core to obtain a green        body, wherein said shell layer is formed from a second plurality        of particles that comprise at least one type of second ceramic        material, wherein said first ceramic material and said second        ceramic material is the same or different; and    -   b. sintering said green body to form a sintered body.

151. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the green body core and green shell layercomprises forming by way of a co-axial nozzle.

152. The method of any preceding or following embodiment/feature/aspect,wherein said second plurality of particles further comprises at leastone pore former or microsphere or both.

153. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a solid core with no central void.

154. The method of any preceding or following embodiment/feature/aspect,wherein said green body core is a hollow core having a central void.

155. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the green body core and green shell layercomprises forming by co-axial extrusion or co-axial spray-drying.

156. A method of making a ceramic proppant comprising:

-   -   a. providing a fugitive spherical core;    -   b. forming at least one green shell layer around said fugitive        spherical core to obtain a green body, wherein said green shell        layer is formed from a plurality of particles that comprise at        least one type of ceramic material; and    -   c. sintering said green body to remove at least a portion of        said fugitive spherical core and form a central void and a        sintered shell body.

157. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core comprises at least one polymer.

158. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is polymer core.

159. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core comprises at least onesilicon-containing polymer.

160. The method of any preceding or following embodiment/feature/aspect,further comprising forming said fugitive spherical core by extrusion orspraying drying.

161. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is a solid core.

162. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is a core with a central void.

163. The method of any preceding or following embodiment/feature/aspect,wherein said forming of the at least one green shell layer comprisesutilizing a fluid bed to apply said plurality of particles to providesaid green shell layer.

164. The method of any preceding or following embodiment/feature/aspect,wherein said plurality of particles further comprises at least one poreformer or microsphere or both.

165. The method of any preceding or following embodiment/feature/aspect,wherein said sintering comprises sintering in an oxidizing atmosphere.

166. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is pyrolyzed during said sintering.

167. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is pyrolyzed during said sinteringand at least a portion of said fugitive spherical core forms a pyrolyzedmaterial that reacts with at least a portion of said green shell layer.

168. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is pyrolyzed during said sinteringand at least a portion of said fugitive spherical core forms a pyrolyzedmaterial that reacts with at least a portion of said green shell layerto form a mullite phase.

169. The method of any preceding or following embodiment/feature/aspect,wherein said fugitive spherical core is pyrolyzed during said sinteringand at least a portion of said fugitive spherical core forms a pyrolyzedmaterial that reacts with at least a portion of said green shell layerto form a mullite phase in a radial region closer to the central voidand wherein a radial region further away from said central void containno mullite phase.

The present invention can include any combination of these variousfeatures or embodiments above and/or below as set forth in sentencesand/or paragraphs. Any combination of disclosed features herein isconsidered part of the present invention and no limitation is intendedwith respect to combinable features.

Applicants specifically incorporate the entire contents of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

1-26. (canceled)
 27. A proppant comprising a plurality of micronparticles that are sintered together, wherein said micron particles havea unimodal particle distribution, wherein said micron particles have ad₅₀ of 0.5 micron to 3.5 microns.
 28. The proppant of claim 27, whereinsaid proppant has a plurality of pores having a pore volume wherein amajority of the pore volume results from interstitial gaps formedbetween the micron particles.
 29. The proppant of claim 27, wherein theproppant is spherical and have a Krumbein sphericity of at least about0.9 and/or a roundness of at least about 0.9.
 30. The proppant of claim28, wherein the pore volume is from about 1% to 30% based upon the totalvolume of said proppant.
 31. The proppant of claim 28, wherein the porevolume is from about 5% to 20% based upon the total volume of saidproppant.
 32. The proppant of claim 27, wherein said proppant has aspecific gravity of from 0.8 to
 4. 33. The proppant of claim 27, whereinsaid proppant has a specific gravity of from about 1 to 3.5.
 34. Theproppant of claim 27, wherein said proppant has a d₁₀ that is within100% of the d₅₀.
 35. The proppant of claim 27, wherein said proppant hasa d₁₀ that is within 50% of the d₅₀.
 36. The proppant of claim 27,wherein said proppant has a d₉₀ that is within 100% of the d₅₀.
 37. Theproppant of claim 27, wherein said proppant has a d₉₀ that is within 50%of the d₅₀.
 38. The proppant of claim 27, wherein said proppant has ad₁₀ that is within 100% of the d₅₀ and has a d₉₀ that is within 100% ofthe d₅₀.
 39. The proppant of claim 27, wherein said proppant has a coreand at least one shell on said core.
 40. The proppant of claim 39,wherein said core comprises said plurality of micron particles that aresintered together.
 41. The proppant of claim 39, wherein said shellcomprises a plurality of micron particles that are sintered together.42. The proppant of claim 27, wherein said proppant is in the absence ofa binder.
 43. The proppant of claim 27, wherein said proppant is in theabsence of a polymer.
 44. The proppant of claim 39, wherein the corecomprises a plurality of micron particles that are sintered together,wherein said micron particles have a bimodal particle distribution witha modal A particle distribution and a modal B particle distribution. 45.The proppant of claim 44, wherein said micron particles of each modalhave a d₅₀ of 0.5 micron to 3.5 microns, and modal A has a d₅₀ that isat least 10% different from the d₅₀ of modal B.
 46. The proppant ofclaim 44, wherein said micron particles of each modal have a d₅₀ of 0.5micron to 3.5 microns, and modal A has a d₅₀ that is at least 20%different from the d₅₀ of modal B.
 47. The proppant of claim 44, whereinsaid micron particles of each modal have a d₅₀ of 0.5 micron to 3.5microns, and modal A has a d₅₀ that is from 10% to 100% different fromthe d₅₀ of modal B.
 48. A proppant comprising a core and a shell,wherein said core is a ceramic or oxide core, and said shell comprisesat least one ceramic material, and said proppant has a core strength toshell strength ratio of from 0.8 to
 1. 49. The proppant of claim 48,wherein said proppant has an overall proppant strength to core strengthratio of from 2 to
 3. 50. The proppant of claim 48, wherein saidproppant has a specific gravity of 2.6 to 4.5.
 51. The proppant of claim48, wherein core is a synthetic core. 52-76. (canceled)
 77. A pluralityof sintered ceramic proppants having a mean particle size, wherein thesintered ceramic proppants are monodispersed with a standard deviationof 3 or less.
 78. The plurality of sintered ceramic proppants of claim77, wherein said standard deviation is 2.75 or less.
 79. The pluralityof sintered ceramic proppants of claim 77, wherein said standarddeviation is 2 or less.
 80. The plurality of sintered ceramic proppantsof claim 77, wherein said standard deviation is 1 or less.
 81. Theplurality of sintered ceramic proppants of claim 77, wherein saidstandard deviation is 0.5 or less.
 82. The plurality of sintered ceramicproppants of claim 77, wherein said standard deviation is from 0.5 to 3.83-169. (canceled)